There's
some wiggle room when it comes to sizing, but in my opinion, not more than a few cm. Get your saddle-to-pedal positioning situated. After that, if you can use a reasonable length stem to put the handlebars in the perfect spot, it's your size. In VERY general terms, subject to much debate, here's a starting point:
* Adjust the seatpost height such that when you're riding, your leg is very slightly bent when the pedal is at the 6 o'clock position. You want leg extension, but if your knee locks or your hips swivel from side to side as you pedal, the saddle's too high.
* Position the saddle fore/aft on the seatpost such that your knee is directly over the pedal spindle when the pedal is at its most forward position. This school of thought on saddle positioning is often abbreviated KOPS (Knee Over Pedal Spindle) and while it's not an absolute, it's a decent and simple to understand starting point. You can read more
HERE on Sheldon Brown's web site.
* Once you've got those worked out, lean into a riding position. Don't lock your elbows. Are you comfortable with your hands on the brake hoods? In the drops? Some people believe that ideal handlebar position will be near the point where the handlebar is visually aligned with the front wheel's hub when you're in a riding position. Like KOPS, it's not an absolute, but the norm will be pretty close. You may need to adjust both the height/angle of the stem and its length to get to this point, but it should be obvious if you're close enough that a reasonable replacement stem will put the bars in the ideal position.
If you can meet those three conditions above, there's a pretty good chance the bike fits. It just so happens that for most of us average-sized people, a bike that fits and is built to typical dimensions will also give you a little clearance when standing flat-footed and straddling the bike. On an older bike with a horizontal top tube, it may be just an inch or so of clearance. Maybe even pretty close to being a little too intimate with your intimates. On more recent bikes, the top tube slopes downward from the front toward the seatpost and you'll have a little more clearance.
If the bike can't be adjusted to fit properly, it won't be a good value for you no matter how cheap it is. After enough miles, "pretty close" can be pretty uncomfortable.
And not that it matters a bit, but I just noticed you're in the East Bay... my old neck o' the woods.