It will copy as many characters as there are in the source string. It has no idea how large the destination buffer is, and will overwrite into random territory if you're giving it a too long source string. There is no "default size" and absolutely no "guessing" involved.
Be careful, it sounds as if you're not very clear on how the basics work here.
If you know the length of the source already, it's better to use memcpy()
.