Riding a bicycle with a frame that doesn't fit your body size and type cuts back on efficiency and comfort. Poorly matched bike frames force riders into awkward postures and limit the power your legs can put into the drive train. Riders with a large build should choose a bike to fit their riding style first, and then find a frame to match.
Measure your inseam length from floor to crotch. Stand upright with your feet as wide apart as the width of your hips. Use a measuring tape to find the distance between the bottom of your pelvic bone to the floor beside your heel.
Mountain bikes designed with smaller frames than road bikes turn sharply and maneuver well on rough trails. On paved roads and city streets or on long rides in the country, mountain bikes won't match the stability and comfort of road bikes with larger frames. Decide what type of riding you will do and choose an appropriate class of bike for that. Manufacturers use different formulas to match riders to these different frame styles, but base the fit on individual height and inseam measurements, according to Schwinn.
Check proper frame height by straddling the bike over the pedals with the bike upright and both feet flat on the ground. Standing up straight you'll need at least an inch of clearance between the top of the frame and your crotch. On bikes with a dropped frame style this measurement isn't necessary.
Pick a frame size using the manufacturer's recommendations. Schwinn recommends a mountain bike frame of 23 inches or more for a rider for a rider 6 feet, 4 inches tall with an inseam of 35 inches or more. Schwinn recommends a road bike frame size from 62 to 64 cm for the same rider -- that's from just less than 24 1/2 inches to 25 1/4 inches. Other manufacturers may use different methods of sizing frames.
Once you've found the best general fit of bike and bike frame, consult with an expert rider to adjust other parts of the bike. Consider stem risers if the bike fits your lower body but forces you to bend more than 50 degrees from vertical, Rivendell Bicycles suggests. Proper form with hands on handlebars while comfortably seated puts the body at an angle between 50 and 65 degrees.