Is there anywhere in the ACP to set a max signature height & width , i cant seem to find it anywhere and ive never recalled seeing an option for doing this.
You can set the max number of characters in a signature but there is not way in the standard mybb to specify max dimensions of an image in the signature.
Try looking for a plugin to do the job.
There is a plugin for it. I am 99% sure I have seen one before, unable to find now though sorry.
Not sure if you still need this or not but i have come across this code which dose work as i am using it on my board.
open the postbit_signature template. Its content should be:
<hr size="1" width="25%" align="left" />
{$post['signature']}
Replace it with
<div class="signature">
{$post['signature']}
</div>
open global.css and add:
.signature img {max-width:600px; max-height:150px; overflow:hidden;}
.signature {border-top: 1px solid #aaaaaa; width:100%; margin-top:1em; padding-top:0.2em; max-height:200px; overflow:hidden;}