@InspectorCaracal @highfellow @switchingsocial @jond max length is a tricky beast. Thr web standards do not define it, so it is left to implementors.
Some browsers take up to around 4000 characters, some even more, but for wide-spread support, you should stay within two-toots-worth (around 1000 characters).
Now, how would mastodon deal with them? Dunno, it might be worth a look into which url parsing library they use and find out.
mastodon.at is a microblogging site that federates with most instances on the Fediverse.