I was wondering if the trunk of an Xd can fit a full size spare tire instead of the donut. thanks.

I know I wouldn't be able to fit one of my tires in the well lol. Not sure. It doesn't look that deep to fit a 16". If you can, then putting that foam cover back on won't be flat.

nope it will not fit thats why scion gives you thats strap to tie down your full size tire when you get a flat

hmmm interesting...a strap?? where is it??

I have yet to see a strap in the back of my xD or anything mentioning a strap :?

Check page 215 of your owners manual it shows you how to pass the strap through the flat tire.
And as for the strap it was in that compartment in front of the spare tire next to the tire iron