Hello, how does the process choose which size of standard cells to use. Is it possible to restrict the process to use only, let's say the smallest size of the cell? or any other size.

You can use the no_synth.cells list for that purpose. The default list is part of open_pdks currently and there is a different list for each STD_CELL_LIBRARY (i.e. https://github.com/RTimothyEdwards/open_pdks/blob/master/sky130/openlane/sky130_fd_sc_hd/no_synth.cells) However, you can find it locally under

$PDK_ROOT/sky130A/libs.tech/openlane/<library>/no_synth.cells

Any cell in that list won't be used in synthesis. You can find more about that here.

Thanks