The 2.5 will give him more room to swim around, IMO, that would be better for the fish. The larger the tank the easier to keep water quality good as well. Be very careful with what u put in the tank. Lots of craft items have varnish and various coatings on them and are not meant to be put in water, esp water that contains fish! In fact, when I worked for a craft store, I noticed they had begun putting "not for aquarium use" on many of the brands of "river rocks" and such, prolly because the coatings on the stones were poison to fish. If u live near a stream, u could find your own. I know they have small Buddha statues for aquariums out there.
If u want to "build" something for the tank, the only adhesive I'd use would be aquarium silicone, or aquarium putty. They are non reactive when dry and meant to be in an aquarium environment. Good Luck!