Hooo, boy!
You're in a pickle. Unfortunately, you need several days to study current price guides, make notes on which models are the most desireable/valuable, see what he's got, make decisions, etc., etc.
There are radios that are worth thousands of dollars. I'm not nearly enough of an expert to be able to give you anything approaching advice, but I'll try to give some basic pointers.
As Sandy said, just like all vintage electronics, the heavier, the more knobs, the more tubes, generally, the more expensive it was when new, and the most valuable today.
One of the most desireable brands was E. H. Scott. Huge, expensive, gorgeous.
Look for radios with colored mirrors as part of the design. Sparton made one called the "Bluebird" that has a round, blue mirror. There are several others. If the mirror is still in perfect condition, could be very valuable.
You may have heard of a type of plastic called "catalin". It was popular for a very short period of time. Usually in very bright, vivid colors, frequently with swirled patters. The right radio, in the right condition, $5,000+++.
Wooden cathedral-shaped or tombstone-shaped radios are very popular, but there are literally hundreds of makes and models, and values range from $25 to as high as you want to go. Certain Philcos, Zeniths, (look for a black dial face) can be valuable. The floor models tended to be the fanciest and most expensive then and now. As you mentioned, they take up a lot of room!
If you want solid advice, the AK of antique radios is
www.antiqueradios.com. They'll be happy to give you great advice, but they'll want to know what makes, model numbers, condition, etc. Otherwise we're all just guessing.
Let us know what you find out!!
Clay