When you're selecting a mat, one of the decisions you'll have to make is simply how big the mat is going to be!
One simple rule of thumb is that the mat should be no smaller than 80% of the width of your front door. Any smaller, and your entryway may look lopsided. Measure your front door's width, and rule out any mats whose widths are less than 4/5 of that number.
If you have double doors, remember to include both doors in that measurement. This may result in a door mat six feet wider - or more! However, remember that it'll keep you covered in the event that either door is used.
As far as depth goes, you'll find that many mats are standardized at about one and a half to two feet. If you have a longer porch entryway, select one on the larger end of that spectrum to help balance out the look of your entryway.