Depends.
You couldn't fit the big leaf on a wedding ring
If you go too small no-one will be able to see it....
If your just practicing drawing do them big, if your designing something you need to figure out the scroll size & what looks best.
Good advice, Walter, especially when your leaf weighs 300 pounds.
Yakob, if you can get a copy of Lee Griffiths's book "Art and Design Fundamentals" or Ron Smith's "Advanced Drawing of scrolls, your questions will be answered.
Meanwhile, draw the scroll backbone to fit the space you wish to fill. Then lightly draw a series of ovals that just fit between the backbone lines of the scroll. Each consecutive oval should just touch the prior oval and just touch the backbone lines. Fill the space between the backbone lines with these ovals from beginning to center. Then just draw a leaf within each oval and the leaves will be the perfect size.