I use transistor to form decouple cap (decap) for power supply vdd.

what are the differences between a single large decap with size 50u x 50u (width x length)  and multple decap with smaller size but same area in total, suh as 25 10u x 10u decap. Which one is better?

The differences from aspect like frequency response / speed, ESR, Rg, leakage etc..

Shorter lengths will be faster, but you might want to lower the Q with longer lengths. I asked the same question myself and finally got it figured out with some help. Click here for the thread.