Anything from a 18 inch wheel to 24 inch would be normal for a 7 year old girl. ( Doesn't see to be any such thing as metric in the world of kids bikes).
You wont know until you put her an a few bikes and see which one she is comfortable with.
Can you put her on a few friends bikes? Are you OK with taking her to a bike shop to try out some bikes, even if you have no intention of buying there ?
As for buying on-line, if you are buying a flatpack bike , do you know how to assemble a bike safely, especially the brakes.
Its not impossible to get it right, but takes a bit of time and practice, and
having safe brakes is essential obviously.