Question

Experimental boundaries for size of electron?

Answer 1

There is some confidence that electron is a perfect point e.g. to simplify QFT calculation...

Answer 2

There are theoretical constraints, not just experimental ones. Some relevant search terms are "preon" and "confinement problem". Experimentally, I think the bound should be no worse than hc/Ehc/E, where EE is the the energy scale probed by the LHC, so about 10−1810−18 m. (There may be a lower experimental bound coming from high-energy cosmic rays, or from high-precision measurements.)