If you want a shortcut, get a really big Halloween costume and modify it. Of course, if you want to make one from scratch...
If you want a basic one (Let's say a green full body monster), you're gonna want to get lots of fabric. It's probably the same basis as a Halloween costume, but if you want one your size, you'll need the assistance of someone else to measure things.
The mouth could differ. It could be a regular hand mouth (Big Bird),but if you're looking for something like that, you'll need a huge puppet head with the mouth close to the top, so your hand could hold the top up while operating the mouth. Or, if you have the materials, stuff the top of the head with foam and put the mouth a little lower.
Also, the bottom waist down is probably part of a two-piece, with the waist up being the top part. Just saying.
As for hands, one hand will have to be stuffed because both of your hands will be busy operating one puppet arm and the mouth. So preferably sew the arm onto the body, and make sure it looks pretty casual.
You'll want to make a starting foam template of the puppet, and add fabric onto it.
Like I said, it'll probably take multiple people, and it will be long and grueling. Hopefully you can put my tips to good use!~