The effects of riding a bike that's too big for you depends on "where" it's too big for you. The standover element of bike fit is easy enough to gage. Can you stand over the top tube and still have about an inch between the tube and your crotch? Assuming that, you should be able to place your saddle so that is higher than your handlebars. How much higher is a function of what you intend to use the bike for, e.g., racing versus recreational rides, your flexibility and reach. If the bike is too "tall" for you, it will be impossible to raise the saddle high enough to put you in an aero position. In effect, you'll be sitting upright like you would on a hybrid or mountain bike.
That said, when people complain that their bike is too big for them, I generally take that to mean that they feel too stretched out, i.e., the top tube plus stem is too long. As a starting place for determining whether the bike you're interested in is too big in this regard, when your hands are in the hoods, you should have some bend in your elbows. Your arms should not be straight and rigid. Riding in this same position, the hub of your front wheel should be obscured by the handlebars. If you're too stretched out, you won't be balanced ideally on the bike and may experience neck, shoulder, arm and/or back pain.
The problem that many riders encounter, however, is that reach is not a static thing. It changes as you grow stronger and more flexible on the bike. When I first bought my bike, I felt just a little too stretched out and experienced some elbow pain at first because my arms were too straight. After 2,000 miles, however, the bike feels great. I may even have to get a longer stem sometime next year.
My advice? Buy a bike from a shop that spends time--ideally a lot of time-- with you on fit. They should, in the very least, take some arm, shoulder and leg measurements to determine a good starting place for you, and once you select a bike, they should spend some time getting your saddle and bars in the "right" position. If all they do is have you stand over the bike, go to a different shop. Or do what I did. I bought a bike from a shop that took measurements of me, bought the bike that best fit me and then took it to a professional fitter for fine tuning.
Good luck,
Kate
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher