How large of a space inside?

It's 37 x37 inches so almost 9feet of space inside. I bought it to stop my dog from peeing on the edge of the pad. Plenty of room for all 3 of my small dogs. Bought reuseable pads that fit fairly well inside. Its nice because i can pick the whole thing up and it folds so place in shower and hose down when i need to. Fir me it was a good pyrchase.

Buy a mat placed at the bottom of the cage is large, what size mat suitable for the base of this cage?

I bought a repeatable machine washable pads, 36" by 36" size put the bottom of the cage is very suitable, puppy pee is also very easy to see, my puppy very much like, highly recommended, I bought this link in Amazon https://www.amazon.com/Urine- Included-Reusable-Waterproof-Absorbing/dp/B0BHJKT88D/ref=sr_1_1?

Is there a bottom that fits this?

You can find fabric washable puppy mats on Amazon that fit this playpen. I have not found a hard plastic that is fitted to this BUT I discovered that if you look up "under sink silicone tray" the 34" wide trays fit PERFECTLY and they have a lip and are super easy to clean. These are meant to line the inside of your sink cabinet incase it ever leaks, they have a holding range or 2-3 gallons. They are usually about 34inx22 in so the space not covered is where I keep my dogs bed and it has saved my floors for over a year.

Style	4 Panel
Item Shape	Rectangular Prism
Color	Black
Is Assembly Required	Yes
Item Dimensions L x W x H	36.06"L x 24.68"W x 3.94"H
Closure Type	Latch
Material Type	Plastic

24" Dog Playpen 4-Panel Pet Exercise Pen with Door Gate 35 x 35 x 24 Puppy Play Pen Indoor Outdoor Plastic Foldable Portable Pet Fence for Small Medium Dogs Puppies Multiple Pets Black

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