what size speakers are in the back of the 2001 suburban

They're 6.75 inch speakers (same as front), which can be hard to find. If I were you I would go to www.crutchfield.com and select car speakers, then use the car selector tool, they also have good deals. If you replace the speakers, then there is a special wiring harness adapter you will need to use to connect them. (In the rear, there will be a plastic mounting plate that the speaker is glued to SAVE the plate, use it to drill the new speaker holes in). Front is just remove, drill, replace.