I pulled my engine and transmission out of my 69 in my backyard.
Had the front supported with jack stands where the lower control arm mounts. I would say the jack stand was about a foot and a half from the bottom to its top where the car would sit.
Then I raised the rear up to make it about level.
Make sure you buy a quality load leveling device. I picked up a cheap one at Pep Boys and it barely worked. It would bind when trying to angle it steep to get the engine and trans out together.
If you are just pulling the transmission without the engine You would need it high enough to get the trans in and out from under the car + whatever room to work. Hard to give you a measurement for this way so I will just say as high as you can get it. Especially if the bell housing is coming out with it, thats the biggest part. If you pull the trans from bellhousing then the bellhousing from the engine (if needed) you wouldnt need to get it as high because you could just flip the housing over on its side so it would aonly be about 7 inches tall vs whatever the diameter of it is a foot or more?