55 gallon tank may be 'larger' but it's actually quite a bit smaller than a 40 long.
Snakes can't be kept in groups in most cases. And they are from the wild, nothing is 'too much' for one.
I suggest a corn snake. They get to a nice size without being too large and are diurnal(active during the day time) and tend to use their entire enclosure. Some have even given a single corn snake an 80 gallon enclosure, even though they don't need anything bigger than a 20 long--they just use it well.
The only thing that could get big enough to look at your chihuahua would be like a burmese python, reticulated python, or a really really big boa. But like you said, a 55 gallon would be far too small.
-Corn snake (day time active, no need for uvb, great at eating frozen thawed rats without being picky, docile, easy care)
-musk turtle (more difficult care, needs UVB and some shallow water. Turtle that stays only 4-6 inches. Real cute. Needs double filtration)
-Live terrarium with living plants and a bunch of anoles (needs uvb, less handleable, but cool to look at)
-Ball python (CAPTIVE BRED, get one already eating frozen thawed, they are picky eaters and wild caught ones are nearly impossible to get to stop eating live)
-red eye croc skink pair(needs humidity, would be a bit trickier)
-Other small snakes (kingsnake, milksnake, sand boa, rosy boa, rat snake)
Biggest recommendation is corn snake. They are a perfect size snake, great colors, cheap, docile, and easy feeders. Active and brave--no where near as easily frightened as a ball python. They will use whatever space you give them. Get one from a breeder instead of a store.