i just bought an accord and love it but, i think the 18 inch wheels
and the low side walls are causing an uncomfortable ride.
if i switch to 17 inch wheels with a higher sidewall, how much more
comfortable will the ride be , if any. and, does changing the wheel size
have negative effects on the car?

2013 accord ex l V6 coupe

 

Yes, switching to 17" wheels and tires with an extra 0.5" of sidewall height will marginally improve the ride. The switch will also improve acceleration and braking, but the improvement will be very marginal; steering response may be marginally worse. One other benefit would be some extra pot-hole (or other road hazard) protection; that said, I have a 17" wheel/tire (235/45 R17) setup on my TL and just destroyed a tire and wheel in a pot hole, so "extra pot-hole protection" is still no guarantee you won't have an issue.