If you search on internet, you will probably find all the kind of opinions about this topic.
However, a lot of design guides recommend to use small decoupling capacitors (the main reason is smaller distance between pads - smaller loops). However, there is also practical reason - placing 0603 under a BGA is very hard and based on many design guides, decoupling capacitors should be placed as close as possible to power pins (again, you can find any kind of opinions on internet telling, that this may not be necessary, however, the designs I do by following this rule always work perfectly and reliably and I am not going to try placing decoupling capacitors far away from the pins before I see recommendations from companies like Intel to do so
So, if you have a BGA on the board you may want to consider 0402 - it can be soldered by hand, no problem, but you may need a microscope. Otherwise, depends on the circuit, but 0603 should work fine in general designs. I have not seen a design failing because 0603 were used.