I have to replace my tires im keeping the 245/35/19 on the front but the rear I want to go wider should I go with 285/35/19 or 285/30/19. I have 275/35/19 on my wheels now.

 

285/35/19 has a closer diameter to stock than 285/30/19.

 

Is the sidewall on the 285/35/19 about the same as the 275/35/19 because I don't want a big sidewall but I don't want a small one like the front either.

 

If you go to a 285/30/19 tire in the rear, your sidewall should be around 3.4 inches. The sidewall on your 275/35/19 tire is likely around 3.8 iches.

 

Use these websites. I use them, when trying to find the overall diameter of a tire size. Comes in handy. The website comes from Bridgestone/Firestone. You can use it to compare the overall diameter of the stock Z tires vs. aftermarket 19's. It's probably not going to help you with the width question though.
http://www.bridgestonetire.com/tires...sproductid=227