Hi - sometimes taping out the area can give you a good visual. Maybe start with a 5 x 8 and go from there. Sometimes the material of the flooring area that you are covering can really also determine the size. For instance if you LOVE the floor and just want to accent it a smaller area rug might suit the bill. If you are waiting to replace the floor, bigger would be better. Hope that helps a bit!

I'd agree with debb but would also add that the function is important as well. In our harsh climate we have a very small rug (2'x3' maybe) sitting right in front of the door so that you can get in with wet boots and take them off immediately. The balance of the area (about 7 x 10) is taken up with a mosaic rug (marble that looks like carpet). We did this because the marble rug is indestructable when covered with wet boots, and the water won't ruin our floors in the entry. The addition of many many many grout lines within the mosaic makes the area non slip as well. I've entered many houses here for parties and with all of the outside shoes piled in the entry, it gets wet and the marble and slate entry's always get really slippery.

This is a summer or early fall picture when we don't have the little carpet in front of the door.

I would not go too big try a 4x6