I think that, for the most part, the definition of a mountain needing to be part of a range is about the most useful one to follow. 'Size isn't everything' and all that

I live at one end of a mountain range which is 'impressive' for reasons other than size. The Strzlecki Ranges are quite small in size, actually. The weathered down remnants of a once mighty mountain range, and the highest peak in the range being Mount Fatigue at 581 metres (1906 ft). Why is that range 'impressive'? Oldest mountain range on Earth, is why!