A couple thoughts for you, Lulu:
1) You can quilt whatever size you want on your machine, but it will really depend on how much patience and effort you want to put into it, and whether you have any physical limitations (arthritis, illnesses, bad back, etc). I do all of my piecing and quilting on a small domestic that has a 6.5" harp and I've free motion quilted king sized quilts, queen quilts, and many twin/lap sized quilts. I am a relatively new quilter, started in 2012, and when I started, I wasn't sure how much I would like it so I didn't want to invest in a super expensive machine only to have it sit there unused. I built a cheap foam insulation topper for my table to fake inset my machine so that the surface is level, and that makes everything much easier. It is definitely a lot of work to wrangle a large quilt through a small machine, but we are limited on space and funds, so this works for me.
2) You can rent time on a longarm. Many quilt shops offer this service where you can purchase a class from them to learn how to load and use their longarm, then you can rent time on it for an hourly fee. That way, the quilt is still made entirely by you, but you're just using someone else's machine. This is what I'm looking at once my tots are bigger, since space is a limitation and the cost is a big factor for us right now. It's $20/hr for me to rent time on the longarm at the shop that I frequent, so you may want to check around for your area.
3) I've read often from members here on the board and other reviews/blogs where there are other issues with owning a longarm that don't crop up with a domestic. Making sure your frame is level is very important. Some people don't have a dead bar on their frame? Tension, just like on a domestic, can be a headache on a longarm. Some longarms are finicky about threads and you can throw off the timing if you hit a very bulky seam. Some longarms require yearly maintenance and some don't. Some require more frequent oiling in multiple spots, and some require oiling in just one or two spots. How the machine feels as you move it is so important, and again if you have any back/shoulder issues, this would be important to know how you feel about doing the motion for hours. You also lose about 5-6" of harp space due to the roller and quilt as you quilt more, so a smaller machine with a 16" harp will only allow you about 10" of quilting space. Most quilt blocks are 12", so it's difficult to do a custom design inside a block without having to advance the quilt.
As others have mentioned, you should definitely try out as many as you can, and for as long as you can. I'm happy to move the quilt, and I glue baste so at least basting isn't as terrible as it could be. I'd love a longarm, but that's not really a possibility for me currently, however much I may dream about it. I plan to rent time on the shop's longarm once life allows, and that will also show me if I really want to own one or not, and if I will use it enough to justify the cost for myself.
Hope that helps!