[Eric]

the size & weight of the 18"s should match the 17"s.. As for the cargo area, I wouldn't worry about that unless you put a box for your sub(s) in the hatch... The back seats fold down and you have a ton of space... If needed, you can also lean the front passenger seat all the way back & you'll have the back of the hatch to the dash

btw, welcome to the site!