Fit and experience
We have eighty people across four offices. Are we too big for you?
We work with businesses from a handful of people up through multi-office organizations with internal IT staff. Somewhere above that there is a size where a dedicated internal department genuinely serves you better, and we will say so rather than take the engagement and struggle.
The questions that actually tell you whether a provider can carry you: what is their largest client by headcount, what is their largest by number of locations, how many technical staff do they employ as opposed to total staff, and what happens when two of your sites have problems simultaneously.
Ask for a reference at your size specifically. A provider whose entire client base is ten-person offices will learn on you, and you will pay for that education.