I guess buying furniture online is no different than purchasing it from a local store in this respect: how will you know your new furniture is going to fit nicely into your room? We discuss this topic alot with customers and often recommend that you use newspapers or string or even cardboard mock-ups to layout your furniture footprints in your room.
We've used online room planners before but the software used to be a little clunky. I found a new online planner yesterday that I like much better. It's called Arrange-A-Room and it's available free courtesy of Better Homes and Gardens.
Arrange-A-Room lets you select the shape of your room, designate the dimensions and then add furniture to it. It's easy and fun! You select which room you're working with (dining room, bedroom, living room or home office) and the software gives you typical furniture pieces to put in the room. You then adjust the dimensions of your furniture pieces, rotate and move them around… and VOILA! You can really get a good idea of how your new furniture will fit into your space.