Here are a few answers to questions we get from many of our customers and new bike shoppers. For mountain bikes, you typically want a bike that gives you 2-4 inches of clearance between the top tube of the bike and your crotch when straddling the bar. This is a good starting point as many bikes have top tubes and stems proportioned to fit the average rider based on this standover rule of thumb. Since everyone is made differently, your local shop can help you adjust your fit with different length stems and saddle adjustments to dial in your bike fit.
Road bikes are a bit more subjective.
In road bike fitting, the question of aerodynamics are of utmost importance. If you aren't going to race, then you need not worry about having a top tube that is long enough to stretch out on. Your position on the bike will be a bit more upright to relieve pressure on your lower back and abs. However, if you do plan to race, you want to have a bike that will allow you to stretch out and get low for greatest efficiency and move back on your saddle for climbing power to the pedals. All three of these offer different feels.