Biggest sheet good size that will fit?

There's a saying: measure twice, cut once. But in your case it's ask someone else to measure and then cut twice, once to fit in the car and once to fit in it's location as a trellis. I'd highly recommend you find another way to get those materials home or install a roof rack or hitch and trailer. Odd are you'll have to cut so much to get it to fit in the car you'll realize later that what little remains won't work and you have to buy another one.

I can fit 10 foot boards and poles in my Prius with front of boards where front right passenger feet go and angles back to back left corner of car and hatch still closes, but that doesn't work for panels wider than a foot or two via reclining front passenger seat.

 

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I don't have an answer to your questions, but if I wanted to move those two items, I'd rent the truck from Home Depot. I just Googled it, and it says $19 for the first 75 minutes.
Just something to consider.

 

great idea, I’d do that myself for sure. But some of us live for challenges.