I'm interested in maybe getting a dojo or two but first need to know how much room they need. I know how big they get, but that doesn't necessarily indicate how many gallons they might need.

I currently have a 55 with three goldfish (and six small tropicals, which I'll address below). I also have a 10 gallon with two small ryukins (will be moved to the 55 in the next few days; they've been happy in the 10 since I got them on Dec. 31, but it's time they move into a bigger tank. The tropicals will go in their old tank). That pretty much fills up the 55, and the 10 will probably be pretty much full, too (I might put in some more tropicals). I want to get a third tank, maybe another 10 or maybe something bigger. Not really sure. What I get and what I put in it depends on how much space dojos need.

Of course, I've never seen dojos in any pet store around here, so it might not even matter.

Dojos are not messy fish, so I would say that they could do with 5 gallons of water each, BUT they love to swim so the biggest concern would the the length of the tank. I wouldn't want to put them in anything that isn't at least 30" - 36" long, so they can rocket around

I think in your 55 gallon having a dojo or 2 would be fine. They are very active!

So then 5 goldfish and a dojo or two in the 55 would be fine? ######, and I was looking for a decent excuse to buy a new tank...

OH! My bad! Ok they really need a BIGGER tank then!
Too funny! I'm working my way to another big tank for my b-day (but I want it early of course) so I can give my fish plenty of extra room and get another fish or 2!
(You could still swing the dojos in your tank while you're shopping for another though if you really wanted to )