when is the right times to use small crank baits over big crank baits...or big cranks over small cranks??

A lot of the time crankbait size is related to diving depth (lipped cranks).  IMO diving depth is the most important consideration when choosing a crankbait so I would look at that first.

A big crankbait (generally) gives off more vibration than a small one, which might help a bass to locate it in off-colored water or at night.

Also you could match the size and profile of your crankbait to whatever the bass are feeding on.  In the fall I usually use bigger cranks because a lot of the young-of-the-year baitfish have either grown up somewhat or gotten eaten.  If the bass are eating slimmer minnow-profile baitfish (ie: not shad) than slimmer bodied cranks like a rapala tail dancer or a jerkbait might be a good choice.  

A lot of the time crankbait size is related to diving depth (lipped cranks).  IMO diving depth is the most important consideration when choosing a crankbait so I would look at that first.

A big crankbait (generally) gives off more vibration than a small one, which might help a bass to locate it in off-colored water or at night.

Also you could match the size and profile of your crankbait to whatever the bass are feeding on.  In the fall I usually use bigger cranks because a lot of the young-of-the-year baitfish have either grown up somewhat or gotten eaten.  If the bass are eating slimmer minnow-profile baitfish (ie: not shad) than slimmer bodied cranks like a rapala tail dancer or a jerkbait might be a good choice.

I think pfisher_d covered it well and very realistically.

Another school-of-thought is to base lure-size on water temperature trend (appetite).

Larger lures during a rising temperature trend and smaller lures during a temperature downtrend.

As an example, smaller lures are generally preferred during a major cold-front (falling water temperatures).

Roger

See above.............