MidWest Homes for PetsQuiet Time Fleece Dog Mat Size: Extra Large (48" L x 30" W)

Q: does it wash well in the washing machine

NO! The description does not state that you should only use a washing machine without an agitator. However the washing instructions say no agitator.

Q: I have a 75 pound lab, which size should I get? 54 x 37? Thank you in advance.

No definitely too big. I realize this is old question but for anyone else looking - most labs and other dogs who like to curl up or "nest" are more comfortable if it isn't too big. My GSD is 85 lbs and loves her 23x36 crate, even though it seems a tad small to me she's very happy. The 42" is big enough for any other than the giant breeds, unless they tend to stretch out.

Q: I am buying the 42" crate & will need to use the divider for our puppy. Does the divider fit on top or do I need a smaller bed size?

because of your question, I pulled out the divider and tried it with the bed in it. Yes, the divider works with the bed in place. The divider connects to the sides of the crate.