How to detect how many characters are in a textbox?

I am writing a very basic program for one of my classes at Cal Poly Pomona

I need a function that will detect and output how many total characters are in a textbox.

Can I use the InStr function? Like, for example

Instr(1, textboxname.text, x*) where x* is something that will detect the end of the textbox characters?

Thanks for reading!