How big are the door openings?

I'd say about a foot in a half. It's working great but getting them out is puzzle.

Does this hold up to rain? Will it rust?

I have not used it outside. In my personal opinion, as it is, it will not hold up to rain; however, I would give it a coat of rustoleum clear coat (comes in a spray). It is a nice containment area and I use it for my rabbits. Love all the access points it has.

Can I use it as cat run, under my supervision? It is an indoor cat and I would like to let him out on the backyard for a bit.

You probably could. But you would need to secure it because a cat could go after something and tumble it over unless you use the prongs they provide to secure it to the ground. It's almost the length of a double wide garage door.

Product Dimensions	330.2 x 101.6 x 101.6 cm; 20.5 kg
Item model number	PGCR-02
Breed Recommendation	Large Breeds
Pet Life Stage	All Life Stages
Closure	Tie
Color	Black
Size	130"×40"
Quantity	1
Special Features	Foldable
Specific Uses	Outdoor
batteries required	No
Item Weight	20.5 Kilograms

PawGiant Chicken Coop Chicken Run Pen for Yard with Cover 130"×40"×40" Outdoor Metal Portable Chicken Tractor Cage Enclosure Crate Outside for Small Animals Duck Rabbit Hen

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