When it comes to mastering, you need to be able to hear super highs and lows. And that's the main problem with using smaller speakers in a smaller room for mastering.
Things will usually translate well across most speakers. But if you're mastering, you need to have full range speakers. Otherwise, you can't master because you can't hear what's going on in the low end and on at the top.
Now, you might be thinking, "But you have really small speakers at the back of the room!" Those speakers are key audio speakers; they are full range speakers, even if they don't look like it. They go mega low and loud, and they are perfect for me to master on because I get to hear a lot of detail that's going on in the mid-range, which is what I want. That way I can hear really good high end with a very smooth open good stereo image, and I can also hear the super low and get those tight.
Most top mastering guys just have one pair of speakers because those are full range; they know them, they know the sound of the room. They know that if it sounds good on those, it's going to sound good everywhere else.
That said, it's as much about having the room set up correctly. If it isn't, you'll likely get this fake level on the top that sounds bright and open in your room, but dull and dreary elsewhere. You also need to have the right size and type of speaker for your room. If you can, get a space that's big enough, with full range monitors. A small room will do, but don't go for massive speakers since you're not going to get a good bass response out of them.
If I were you, I'd be thinking about a two-way with a sub. It gives you that extra lower octave, doesn't get involved with any of the lower frequencies, and you can turn it off. You're going to need some room correction software, though, because it's going to be a bit all over the place.
Before asking somebody to master your stuff, make sure that you can see exactly what I've just said above. They've got a room that's made for the size of the speakers, and they have full range monitors. That lets you know that when it plays everywhere else, it will play fine.