My biggest part to print is 6.5" long. It is possible to print this on the Mini, or is the 6.2" bed size an absolute maximum?
If it is narrow enough you can rotate it and print between corners of the bed.
No, is 4.5 x 6.5 inch.
Better get a Taz!!
You can cut the piece in two and glue it together. If you are printing in ABS, I would put it together with acetone. Acetone melts abs, so applying acetone to two abs surfaces and pressing them together forms a very strong weld. Strong to the point where if the part breaks, it will likely break somewhere else first.
I find the easiest splitting tool out there is netfabb basic:
http://www.netfabb.com/
It is really nice for simple cutting jobs. Hit up youtube for a quick tutorial.
Well it’s already cut in half, so smaller would be too much work. And it is ABS now, but wanting to use PET to stop warping (thus the need for the Mini’s high-temp head).
And already use Netfabb 
Sometimes bed sizes are under-stated, because they don’t want you using the very edge; but I guess the Mini is maxed out.
The numbers we have on the spec sheet should be the same numbers that are hard coded into the Cura bed size. You probably can only get a couple mm more on the Mini.