Not quite sure what you mean here - please explain further.

Keil works by putting a LJMP to the ISR in the appropriate vector location - so the compiler imposes no specific limit on the size of your ISR.

I dunno if Keil C51 is smart enough to spot when you have a really tiny ISR, and place it "in-line" at the vector location?

On the other hand, Keil C51 will place code at unused locations in the interrupt vector table - so the usual advice to always have a handler for All interrupt sources applies!