There is no standard other than, it's called Sheet Music, not music sheets, or in this case, given they're blank, Manuscript Paper. 10 and 12 Stave Manuscript Paper is usually on standard US Letter size paper which is 8.5 x 11 inches or A4. But when Engravers engrave music, they will adjust the number of staves, and the distance between them, based on the need of the music. They will also enlarge or decrease the size of the music so that more or less music will fit on a page. Piano constantly changes spacing because the notes may go above or below the staff many ledger lines - that includes between the two staves of a Grand Staff. The best you're going to be able to do is find something that works for you and what you're writing. 4 Grand Staff systems on a page would be a general nice balance, with them already grouped with the Curly Brace and the two staves slightly closer together for each pair, but with a little more space between each system than it would be if it were just 8 blank staves. That size should also not be so small you get cramps writing in little teensy noteheads! However, I'd also recommend that the better course of action in the long run would be to learn how to use a program like MuseScore and just do it all on computer, and you can adjust the spacing on a per-page and per-system basis as needed. Or by the already made Manuscript Paper books as others have suggested. More on reddit.com

Blanksheetmusic.net you can customize it however you want and print it out for free More on reddit.com

For just day to day stuff, the Proline manuscript paper books are sorta my standard. Only a few bucks, pages tear out pretty easily for moving stuff around, etc. For one special project, I used an Archives spiral bound book . The pages were a nice weight and I felt official, although the 10 staves per page we're kinda weird for me, I'm used to 12. More on reddit.com