I've been searching for an answer to disciphering reel sizes. Currently, I have 4 Mitchell 300Pro's and they are rated by Mitchell (Pure Fishing) as being a 4000 size reel. Reel sizing seems to be an ambiguous rating by the manufacturers. If I were to use another reel to replace the Mitchell, I'd be guessing that a 3000-4000 size would be a starting point. I use 20# braid on all the 360Pro's.....

Can anyone clear the smoke?.........

There is no standard, other than generally smaller number = smaller reel.  The only way to get a sense of one brand versus the other is to ask around online to find someone with both reels who is willing to take a picture or find two people each with one reel and get them to take a picture next to a commonly sized identifier. 

And then you get curve balls like the Shimano Sahara 2500 and the 3000.  Same reel exactly except the 3000 has a little larger capacity spool.  I now have both having picked up a Sahara C3000HG 6.2:1 yesterday with a reel trade in at our Cabelas.  At least new to me are the indicators on the spool for stopping the backing line and the max amount of line on the spool.  Very helpful.

The line capacity spec should give you a clue.  Add in frame material and overall weight, and you can get an idea of form factor.  In a general sense, a 2000-3000 size Daiwa, 2500-3000 size Shimano, 25-30 size Quantum or Pure Fishing brands will all work for bass fishing.

Thanks! I think the "rating" pertaining to the size on my Mitchells is a bit generous at 4000. Regardless, it works and serves me well....