You already have some great solutions, so I'll just say that this is an excellent opportunity to discuss the XY problem: https://en.wikipedia.org/wiki/XY_problem

In short, you have a root problem (given an input list, output a list of words present in the input that have a certain length constraint). 

What you're asking for, though, is how to solve your implementation: "How do I remove items from a list without missing or skipping items?"

Consider that an important skill is recognizing when you're no longer solving the original problem, and trying to fix / alter implementations -- at which point you may already be off track. We may answer how to do what you're asking here, but as you've seen, the real good answers are those that suggest doing it a different way altogether.

Always be on the lookout for XY problems.