While most people that can help might guess you are working in C++ (cin being a function associated with it more than most other things I can think of offhand) you may still wish to state it.
If you are writing an emulator, typically considered one of the harder things you can do as a programmer, I would have hoped you already covered input string sanitisation. That said if it is the sort of thing that motivates you to learn programming then so be it, same deal if it is a simpler system like chip8.
When you say restriction on the strings do you mean at the input (think the classic enter characters and then when it hits the limit it changes the last entered value) or just I don't need buffer overflows? For the former then you are on your own for that one (it is not hard though) but for the latter then what is the matter with a basic pre parser like strlen?
http://www.cplusplus.com/reference/cstring/strlen/
You then just do a strlen on the input, then say if the result <10 print a string saying how about trying something shorter, else call the parser.
Option two. Chop it off. Not ideal as an error message is better than some unexpected behaviour because a command got cut short, however if it was only for something that did not get parsed then it is easier.