The biggest difference in size will come from the bass drum. For a standard drumkit, e.g. the Target, you can expect to have 16 inches of bass drum in front of you. But the RT is half that depth. However, the rest of the parts will take up the same amount of floor space. It's a 14" wide floor tom.
I'll draw a diagram to illustrate!
Edit:
Above is Target, below is the Rhythm Traveller.
The toms for Target and Rhythm Traveller will be the same size, unless you decide to get the version of the Target with the 16" floor tom. But even if you do, that'll only add 2 inches to the size, which isn't that big a difference.
Also, snare size and mounted tom size don't make much difference because they're in the middle.
The difference in hi-hat size makes only 1 inch difference to the overall size.
So, the width is almost the same, maybe about 3 or 4 inches different.
However, the depth is 8 inch different; if you use the 16" floor tom, it'll be 10 inch deeper for the Target.
This is actually the biggest problem. The width of the drumset doesn't matter that much since a wide drumset can be spread out against the wall or something. But when it's longer/deeper, it'll "stick out". But you can jam it into the corner of your room.
The other advantage the Rhythm Traveller has is the tom depth: it's shallower. So it'll be easier to position and can overlap each other. And it's lighter and easier to carry around. This one, you'll have to think about it. But sound-wise, I'll very much prefer the Target. Single-headed toms sound a bit "hollow" to me. You won't have the whole depth of tone.