Some Basic Characteristics:
1) Size- width of the blade between edge and spine (usually between 4/8 and 8/8) I personally like the weight and feel if a 5/8.
2) Grind- how much steel is ground off to create the shape (when you look at the toe end of the blade so your line of sight lines up with the spine and edge, the width reveals a larger triangle full wedge or a small triangle with a super thin majority of the blade. The super thin would be a full hollow grind). Usually put in terms of how hollow the grind is, you will see wedge, 3/4 hollow, 1/2 hollow, 1/4 hollow or full hollow grinds. The fuller the grind, the more you will need to hone to get the desired edge, but it may last a bit longer than the full hollow. I like the half hollow and 1/4 hollow.
3) Point- refers to the toe side (furthest from the "hinge") and its shape. Usually you will see square point, spanish point (when the edge tip is extended out past the spine), or round point (when the tip is slightly rounded off.) Round tip is always best for any beginner. I prefer the round tip myself as it helps avoid nicks a lot more while grasping the art.

Most overall lengths are similar when comparing the overall length of the closed razor. When buying a razor always ask the seller about pitting (little dimples in the steel that can be a pain or even a death sentance when restoring), scrathes on the blade, uneven hone wear and rust on the blade/scales. Check scales for cracks and rust by the pins/wasers. Also, the blade should sit somewhat tight in the scales to aid in honing/stropping. It should be centered in between the scales when closed. Scales can be damaged in a sense that they are bowed or bent.
The classifieds here should be ok, but on ebay many sellers don't know about much of that. They tend to post unclear or poor clarity pics to mislead the buyer. When in doubt ask to see more pics of the razor. Minor pitting isn't a big deal if the price is right, as long as the pits aren't near the edge or honewear on the spine.
Hope this helps. The Wiki section in the beginners section has a small list of razors to avoid and those that are pretty good. Definately check that out. Happy I could help. Hopefully it did
-Dan