In the current Artifact Binding manuscript, the artifact is being defined as a string shorter than 400 bytes. Some people asked why 400 and not 512, which is the limit of some mobile browsers?

The answer is that we use 80 bytes in the fixed string:

?openid.ns=http://specs.openid.net/auth/2.0&openid.mode=art_res&openid.artifact=

Suppose we use 400 bytes in Artifact. Then, the total is 480 bytes.
That leaves 32 bytes to the non-query portion of the URL.