As to size, well, sure, print as large as you want, view it on-screen at a large size and it should give you an idea as to how a large print should look! Just realize that when viewing a print you will typically be farther away than if you are up close and squinting at your monitor!
As to where to have it printed, well, my "first" recommendation is to whether you have a Costco nearby, since if you do, they are a highly recommended "local" choice.
If not, well, I recommend Adorama as a lab. They have excellent quality and excellent customer service. There are other outlets, people can chime in, but I've become a "fan" of AdoramaPix!