It depends on your writing. I'm sure there's something about it in some Gregg Writer and if I find it, I will post it. But for now, you want to make sure they're long enough so you can read the shorthand and know it's an m-n blend.
For example, when you write "man" you spell it out completely. Then write "men" under it, where you omit the 'e' in the shorthand outline. "Man" will be a tad longer due to the "a". However you want "men" to be longer then just the "m" in "man".
Practice writing 'man' and under that 'men' do a few lines alternating each. I just tried but can't get a copy as my phone is down. Anyway, then try a few other words with M in it.
Going through some of the posts, I found this one and remembered I made a penmanship drill for these letters. I think I did that because I needed more practice on them. It gets easy to not pay attention to how long a symbol should be.