Inability to stand over the top tube would be a biggie if a bike were too large for me (ouch!) You definitely shouldn't have to slant the bike to stand over the top tube with both feet on the ground.
But assuming you can do that, if you can't ride on the hoods comfortably (on a drop-bar bike), if your elbows are locked to reach them, you're going to end up with neck, shoulder, and/or back pain. Make sure to test your reach to the bars after getting your saddle fore/aft placement set up. No fair pulling your saddle way forward on the rails to reach the bars, since then you rob yourself of climbing power on the bike and will likely hurt your knees too. Most fitters think that your knee should be immediately above or slightly behind the pedal spindle when your foot is in the front pedal position (3 o'clock). For most women, that results in a saddle that is pushed fairly far back on the rails, since we tend to have longer femurs. And since we also have shorter arms than men, pushing the saddle back lengthens the reach to the bars and may make it apparent that the top tube is too long.
Other issues with a too-large bike: like Eden mentions, if the cranks are too long, you could end up with knee or hip pain. Too-wide handlebars can lead to shoulder/neck pain. But the top tube length is the most important measurement where being too long can make for a miserable ride and after-ride experience.
Some people are much more flexible than others in the back and hips, so you might be able to ride a "longer" bike for your height than most of us could. The important thing to pay attention to is whether you have pain and whether you can control the bike. If the top tube is just a little on the long side, a shorter stem can help with reach issues, but on some bikes, you'll sacrifice some handling that way (the front end will become squirrely). If you're needing to ride a stem shorter than about 7-8 cm, the top tube is likely too long for you.
Oh, and if they measured you at a 43 cm bike and you're 5'4", I agree that that sounds too small. I have ridden a 44.5 cm and currently ride a 46 cm road bike (and a 15.5" mountain bike) and I am only 5'2.5". Just like you say, I think at your height you'd be a 48cm to 50cm (possibly 52cm if the bike has a short enough top tube) kind of gal on a road bike, but it all depends on how the frame is measured and how long the top tube is.
Good luck!
Emily
Last edited by emily_in_nc; 01-05-2007 at 04:52 AM.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow