Did you place it in the room? Making sure there is enough space to move around with the furniture.

Is it hard to walk all the way around it without bumping into other people's knees when the seating is occupied? Do you have to rearrange other furniture to accommodate the coffee table? Step back and take a good look and decide if it looks awkward. You'll know. :)

It depends on size of sofa and if it's a sectional or not. Round or square works better with sectionals or groupings. 24" usually good for average rooms. If more open bigger room then can go 36" sometimes using two smaller tables or ottomans works better. A pic of room you are trying to do would help.

A coffee table is usually a less important, smaller piece than the sofa or chairs it sits with. Make sure the sofa users can get good access to one end or the other of the sofa before they are obstructed by the coffee table (in other words, the coffee table should usually be 3' or so shorter than its sofa). Coffee tables should usually be about 15-18" away from a sofa, by the way. You want to be able to walk between them, and also reach your tea!