Is there an example I can see?

You CAN go in and delete/change a users' avatar or sig in Admin > Characters and Accounts > Website Accounts.

His image is 150x150. I'd like to have a site option/code to limit them to 100x100. I know I can do them individually but if I have to do that repeatedly, it'll be a pain. Also, there's nothing to stop a member from changing it back.

I see.

I know you're comfortable tweaking the Forum Advanced Layout Options. In that case, look into tweaking the avatar sizes in there.

Particularly, Avatars, when uploaded, are pre-resized into 4 or 5 different sizes.

(brb checking numbers)

Alrighty, so the different avatar sizes you can use are as follows:

Thread:Avatar = 150
Thread:BigAvatar = 100
Thread:MediumAvatar = 80
Thread:SmallAvatar = 60
Thread:TinyAvatar = 40
Thread:VeryTinyAvatar = 20

So there you have it. Replace <!--Thread:Avatar--> with <!--Thread:BigAvatar--> (and it'll make your avatars are max out at 100px)