I recommend going no smaller than four spaces. I think it would be useful to have your RBI in the same rack with your Thunderfunk. Sure, you won't always use it, but it's there when you feel like it. And with the extra space you could add a tuner or power conditioner or whatever.
Something else to consider: I actually own *two* racks, a four and a six. These are two independent gig rigs (each has its own power amp and preamp), I don't use 'em both at once. I try to use my four-space as my "go-to" rack, but it's nice to have the six just in case I feel like using more gear.
FYI: the four space has my favorite preamp, a compressor, and a 2-space power amp. The six space actually has 5 "usable" spaces, because I don't count the Furman PL-Plus power conditioner that sits in the top space. Right now the sixer has my second-favorite preamp, a parametric EQ, a 2-space power amp, and an empty space (in addition to the Furman). For awhile I was just using the sixer: power strip, preamp, compressor, parametric, and 2-space power amp.
You have an advantage in that your Thunderfunk contains both preamp and power amp in a 2-space unit, so maybe you won't need more than 4 spaces.