I think it depends on the gas tank you're putting it on. And your budget, of course. The tank Dan and I made for the Indian Hiawatha uses the garden hose hardware; a 3/4" brass hose coupling with one half ground down and a brass hose cap drilled for a vent. On my tank there wouldn't have been room for anything much bigger, so in my opinion it looks good and is proportional. On a tank with more surface area it would look dinky. Price is sure right, coming in at around seven or eight bucks total, a real bargainfor solid brass..
But CCC is right, a small filler takes more caution and time to fill. I made a special filler tube for my antique gas can, soldering a couple inches long quarter inch copper pipe "extension" into the cap itself. It makes it easier to control the filling and still see into the tank for progress to avoid over filling. I used a quarter inch brass bolt to seal off the end of the copper pipe extension. It works and I like it.
SB