It is perfectly normal for rose flower size (and quantity!) to shrink in the summer with a lot of heat. Some roses do this more than others--which varieties do you have? People slow down in the heat of summer, and so do many roses.

Cooler weather will bring bigger flowers once again.

Hoov is right on. Many roses actually stop flowering when the temps rise too far. Significant reductions in size, quantity and frequency of flowering are totally normal. Unless you can cool it off dramatically, the best you can do is keep them well watered and wait it out. Kim

Ditto. Hot, hot, hot. I have been successful in pushing additionl water to get bloom size up during mid-summer. Otherwise, I deadhead, fertilize, etc., and wait for the fall flush.

Besides shrinking in size they can also shrink in petal count and change in color from high heat, drought or even too much rain. Cold can have similar effects too. I think every rose has it's own optimum temps and weather conditions. In some respects they are divas!