If your compilers is any good it will inline, and do a better job than you of figuring out what should be inlined. For that matter function calls are generally fast so long as the objects you copy as part of the function call are not slow to copy (which they can be). There are exceptions to the above, but in general small functions are not a problem.

What is a problem is large functions. I have seen functions that were over 60,000 lines long (and few comments or other excess space takers). I will take a 5 lines max rule for functions (this is nearly straw man levels of short!) over that. Functions that are 50 lines long start to get annoying to read but are not a problem. Even 100 lines functions I can handle. However the extreme of long functions is much worse than the extreme of short.