Here is the section (4.8) that is applicable to ramps...if you do get a service organization to build for you, it may help to have a hard copy of this section printed out so that they know they are building it correctly.
Yours should be fairly straightforward. While 36" is the minimum width, 46-48" is better. Due to your low rise (5") you will not need handrails on each side...only a curb (minimum 2" high) to keep your chair from rolling off of the ramp.
Steve already mentioned the slope, a maximum of 1:12, so for your 5" rise the ramp would have to be a minimum of 5' long. If you want a level landing at the top so you're not trying to umnlock your door while on the slope, a 48" square landing works nicely.
If it's build out of wood, due to the ground contact have them build the structure out of pressure treated lumber. This wood is somewhat unstable, so it's best to assemble the framework with decking screws. NOT drywall screws! Decking screws will hold the wood better than nails when the wood tries to twist due to moisture content fluctuations.
Don't forget a non-skid surface on the decking.
Don't forget the Girl Scouts (I'm an equal-opportunity Cub Scout and Girl Scout leader) or the high school wood shop class...or a church group?
If those sources fail, call your local Rotary Club. They're often the focal point for community service in most towns, they should be able to get the job done or give you a list of contacts.