Describing your firm as “full-service” doesn't work like you think it does.
It would be better to say nothing at all. Listing generic services is not helpful either. Broad and nebulous definitions for your firm hold no meaning and clients can easily dismiss you as unlikely to solve their problem. If you could have solved their problem, they reason, you would have mentioned it.
Instead of being silent, your firm should strive to clearly state that you’re are particularly skilled in managing tight timelines on incredibly large municipal transportation projects; or able to deliver innovative and efficient lighting designs for elementary schools; or your experts integrate renewable energy systems into existing mechanical infrastructure for universities. Each of the previous examples defines — to a granular level — how a professional service firm can use positioning to help clients select your firm.
Simply put, positioning is defining your niche in the marketplace so that potential clients can qualify or disqualify your firm based on their need. It's your responsibility to make sure clients understand that your firm is not merely one of the experts that can solve their problem — but your firm is the only one.
Need a more detailed example of positioning?
Let's look at airlines.