A good technique I've found is to compare your fight to a sports match with similar participants. Examples:
1) One on one fight. Boxing, martial arts, wrestling: pick any. Typically fought in a 20' square
2) Fight between five PCs and five monsters. That's a basketball match. A basketball court is about 90'x50'
3) Party versus four large creatures. That's a polo match. A polo pitch is 300'x150'.
Since these pitch sizes are there to give maximum manoeuvrability to both sides, just about any combat, even on the exagerated scales used in D&D, is going to feel cramped.