When sitting at the piano should the knees go underneath the keyboard? If so, if one has long legs the pedals become a problem. Is there a standard height from floor to keyboard. Being very tall I find this a problem.
I'm 6'3", and my knees are almost never directly underneath the keyboard when I play piano; I think if they were, then my body would be so close to the keyboard that I would feel like a piano-playing Tyrannosaurus. I scoot the bench back far enough for my arms to feel comfortable and then position my legs; they usually end up forming a 60- to 80-degree angle with the ground so my feet can still reach the pedals.
Floor to top of the white keys is fairly standardised at about 28". You could block the whole piano up, but then there would be problems with the pedals. I guess you've got long arms to match your long legs?
Here's one make that seems to offer a tall piano. But I don't know if it would be sensible to get used to one height at home, then have to perform on a lower height when it really matters?
Yes, the knees are supposed to go beneath the keyboard.
Sometimes, leg dollies are placed underneath the legs of a grand piano, to make the instrument easier to roll around. You may find a set that adds an inch or two of height for you.