diff --git a/FLEDGE_browser_bidding_and_auction_API.md b/FLEDGE_browser_bidding_and_auction_API.md
index 4b6c7b014..bdbc3e471 100644
--- a/FLEDGE_browser_bidding_and_auction_API.md
+++ b/FLEDGE_browser_bidding_and_auction_API.md
@@ -17,13 +17,33 @@ const auctionBlob = navigator.getInterestGroupAdAuctionData({
   // ‘seller’ works the same as for runAdAuction.
   'seller': 'https://www.example-ssp.com',
   // 'coordinatorOrigin' of the TEE coordinator, defaults to
-  //'https://publickeyservice.gcp.privacysandboxservices.com' (for now). Used to
-  // select which key to use for the encrypted blob. Will eventually be
-  // required.
+  //'https://publickeyservice.pa.gcp.privacysandboxservices.com/' (for now). Used to
+  // select which coordinator to use for fetching the key used to encrypt the request
+  // blob. Will eventually be required.
   'coordinatorOrigin':
-    'https://publickeyservice.gcp.privacysandboxservices.com',
+    'https://publickeyservice.pa.gcp.privacysandboxservices.com/',
+  // 'requestSize' the affects the size of the returned request (optional).
+  'requestSize': 51200,
+  // 'perBuyerConfig' specifies per-buyer options for size optimizations when
+  // constructing the blob (optional).
+  'perBuyerConfig': {
+    'https://buyer1.origin.example.com': {
+      // 'targetSize' specifies the size of the blob to devote to this buyer
+      // (optional).
+      "targetSize": 8192,
+    },
+    'https://buyer2.origin.example.com': {}
+  }
 });
 ```
+
+The `seller` field will be checked to ensure it matches the `seller` specified
+in the auction configuration passed to `runAdAuction()` with the response. The
+`coordinatorOrigin` selects which set of TEE keys should be used to encrypt this
+request. The `coordinatorOrigin` must be a coordinator that is known to Chrome.
+The `requestSize` and `perBuyerConfig` fields are described in more detail in
+the [Request Size and Configuration](#request-size-and-configuration) section below.
+
 The returned `auctionBlob` is a Promise that will resolve to an `AdAuctionData` object. This object contains `requestId` and `request` fields.
 The `requestId` contains a UUID that needs to be presented to `runAdAuction()` along with the response.
 The `request` field is a `Uint8Array` containing the information needed for the [ProtectedAudienceInput](https://github.com/privacysandbox/fledge-docs/blob/main/bidding_auction_services_api.md#protectedaudienceinput) in a `SelectAd` B&A call,
@@ -130,6 +150,47 @@ const myAuctionConfig = {
 Note that since the `serverResponse` field in the config is a promise it is
 possible to run the on-device auction in parallel with the B&A auction.
 
+## Request Size and Configuration
+
+### Request Size Controls
+The `requestSize` field provided to `navigator.getInterestGroupAdAuctionData()`
+can be used to specify the maximum size of the returned request. If the
+`perBuyerConfig` field is not present and the blob fits into a
+size bucket smaller than `requestSize` then that size will be used instead.
+
+If the `perBuyerConfig` field is specified and non-empty, the returned encrypted
+blob will be exactly `requestSize` bytes long unless there was an error. If an error
+occured then the returned blob will be 0 size and the function may throw an error.
+
+### Buyer Controls
+If the `perBuyerConfig` field is specified then the blob will only include the
+interest groups for the buyers specifically listed. If `requestSize` is not
+specified then the sum of the `targetSize` of each buyer will be used for the
+`requestSize`. Interest groups for each buyer will be added to the request -- in
+decreasing order by the interest group's `priority` field -- until the next
+interest group does not fit.
+
+Space can be allocated between different buyers in several different modes, depending
+on what options are specified in the `perBuyerConfig`:
+
+1. Equally - Space is divided equally between buyers when `targetSize` is not specified.
+
+2. Proportionally - Space is assigned to buyers using `targetSize` as the
+   weight. So a buyer with a `targetSize` of 2 will can use twice as much space
+   as a buyer with a `targetSize` of 1. If all buyers have a `targetSize`
+   specified then proportional mode is used.
+
+3. Fixed/Mixed - Space is first allocated to buyers with `targetSize` specified.
+   They get up to `targetSize` bytes. The remaining space is divided equally
+   between the buyers that did not have `targetSize` specified.
+
+The browser will first process buyers with `targetSize` specified before
+processing buyers without that field specified. Within either of these groups
+buyers are processed in random order. Space that was allocated to a buyer but
+was not used is divided up among remaining buyers based on the active allocation
+mode. The browser may, as an optimization, calculate and use the maximum length
+used by a buyer to better inform its allocation strategy.
+
 ## Privacy Considerations
 
 The blobs sent to and received from the B&A servers can contain data that could be used to re-identify the user across different web sites. To prevent this data from being used to join the user’s cross-site identities, the data is encrypted with public keys whose corresponding private keys are only shared with B&A server instances running in TEEs and running public open-source binaries known to prevent cross-site identity joins (e.g. by preventing logging or other activities which might permit such joins).