How do I set the size of a SF Symbol in SwiftUI?

You can set weights and sizes:

Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .ultraLight))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .thin))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .light))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .regular))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .medium))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .semibold))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .bold))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .heavy))
Image(systemName: "checkmark.circle")
    .font(.system(size: 16, weight: .black))

SF Symbols are similar to fonts, thus:

.font(.system(size: 60))

An alternative is to use .imageScale().

Image(systemName: "chevron.left").imageScale(.small)
Image(systemName: "chevron.left").imageScale(.medium)
Image(systemName: "chevron.left").imageScale(.large)

If you want to use frame you can as well:

Image(systemName: "plus")
     .resizable()
     .scaledToFit()
     .frame(width: 24, height: 24)