Minimum plate size for joist bearing on masonry

The minimum should be in your joist catalog or in the joist spec. If it works for bearing on the masonry and the thickness is sized correctly I see no reason to make it bigger. Our typical size is a 5x8 I think, that does give more area than the 6x6. You want wider so that there is less required precision in the plate placement. If it's off by an inch or two it won't kill you.

No rule of thumb needed - the Steel Joist Institute states the (nominal) minimum requirements in their specification. You can download the specification (free .pdf version), "41st Standard Specifications & Load Tables For Steel Joists & Joist Girders" from this link
You will have to register to get it, but I have not seen any "consequences" to registration since doing so last year.

For K-Series: 4"x6", see page 17

For LH- & DLH- Series: 6"x9", see page 48

For Joist Girders: 6"x9", see page 67

The bearing stress on the masonry & plate thickness have to be determined in accordance with AISC (ASD) requirements. Smaller bearing plates are allowed with certain restrictions.