Homemade bike ramps are relatively easy to build, using two-by-fours for supports and bracing and 3/8-inch plywood for the ramp surface. Construction is simple once you choose how long and wide to make your ramp and whether to use straight or curved board for the ramp surface.
Straight vs. Curved
The two basic options for a homemade bike ramp are having it straight and having it curved. If you're building a smaller ramp and aren't especially concerned with the amount of air you can get (if you're building the ramp for a younger child, perhaps), you can get by with a straight ramp, which is easier to build. But curved ramps, which are more difficult to build, offer greater height and greater slope on a shorter takeoff and are better for more advanced bikers.
Standard width for a bike ramp is about four feet. If you're building a professional-scale giant jump, you may want to go wider, but such construction is beyond most amateurs. For smaller ramps for kids, two feet is more proportional, but giving the ramp a little extra for safety is not a bad idea. In general, most ramps will be just right with four feet of width.
Length and Stress
The length of the ramp can bring some more tricky construction questions into play. A shorter ramp will have a greater width-length ratio and is actually more vulnerable to stress down the center line. For shorter ramps (six feet or less), you should use three support rails, contoured to the shape of the ramp, one on each side and one in the middle.
Length and Flexibility
Short ramps have another disadvantage: The ramp section is actually less flexible when it's shorter (think of how it's harder to break a short stick than a long one). Shorter ramps often need partial cuts spaced an inch or two apart along the bottom of the ramp to help the ramp board flex into shape on a curved ramp. Longer ramps can simply be pressed into shape.