Tanks size does make a difference. I currently have my Syrian in a 29 gallon tank, and wouldn't recommend anything smaller, length wise. Even though its a fairly big tank for fish, it feels crowded with all the necessary hamster accoutrements in it. If I had the space and money for a bigger tank and stand, I would upgrade in a heartbeat.
I don't really think that with any species, besides Chinese hams, that a topper would work. Chinese are so mouse-like, and able to climb so well that they would be able to easily navigate a topper, unlike the other, much more clumsy species. So it really depends on the species of hamster you want to get.
If you're going to go the route of something other than a Chinese, than probably going for more floor space, rather than height, is the way to go. I found that added little mini levels inside the tank is much better for my Syrian ham. If they do fall or jump off, they can only go about 5 inches or so down, and they have a 4-5 inch thick bed of Carefresh to land on.
Oh, I probably should have described what I used as a mini level. I found a shower caddy with suction cups that stuck to the sides of the tank and made a nice corner shelf/mini level. I have it placed about 4 inches above the top of the Carefresh and it leaves enough space underneath it to place a hidey, or some other item.
It was this caddy:
Amazon.com: InterDesign Large Suction Corner Shelf, Clear: Home & Kitchen
To make it easily accessible to the ham, I fashioned a ramp out of a 7 rung tall bird ladder, with some craft/popsicle sticks glued to one side to make a sort of solid floor beneath the rungs. I just placed it at a nice easy angle for the ham to be able to walk up, without trouble. I'll upload pics of the whole set-up tomorrow, when I am more awake and can find the camera.
I use the corner shelf caddy as a sort of feeding area. I put the ham's food and treat dish there, and have a hook hanging above the area to put treat sticks on. I find that it helps to keep things neat a tidy, and prevent the ham from burying its food dish under the substrate thus preventing me from seeing how much food is in it, and whether or not I need to add more.