"I use the Keil C51 V5.02 ... C programs should need only about 5..20% more code space as ... assembler."

Note that v5.02 is a pretty old version of C51; newer versions should offer (significant) improvements on these figures.
(but YMMV, as the Americans say)

If you're just starting out with 'C' for the 8051, there is no reason not to start with the latest tools!