Command usage: sizeup <mobname>
The command compares you to the mob in question and gives you an output of that comparison. Then you can choose whether to attack or not.
There are, however, some caveats. The command references the Appraise skill (who else has wanted a purpose for that skill forever?), on a graduating level based on your player level. If your skill is low, you will receive 'failure' messages. There are also success messages. It is your responsibility to interpret these messages. We cannot give everything away or the game becomes worthless, so the command will not be 'errorproof':
USE AT YOUR OWN DISCRETION
Thanks go to Disdain for thoroughly testing the command for me and providing feedback on bugs and quality concerns.
He claims to find the command quite useful--it never steered him wrong (Dis, Jan, 2012). So give it a shot (and let me know what you think)!!!