Hmm, I find that even the biggest one they sell fits in my trunk. You should find one that fits snugly in yours. It helps a lot when everything is not rolling around losing their liquids lol
I have 4 tupperware boxes. 2 for Pads (6.5in in one, 4in in the other), 1 for holding my MF towels, and that huge one that usually stays in my trunk for mobile/whatever I need it purposes.
Quite useful for anything you need. Especially if there's liquids that can spill out then you are covered from it soaking into your carpet