Hi,
At first sight this is a problem of dissipated power.
For DC currents this is I x I x R_Cu.
But it isn't that simple.
It additionally depends on:
* core dimensions
* isolation material
* frequency (skin effect)
* ambient temperature
* coil stacking
* RMS current
* additional heat sources (like core loss...)
* local safety regulations (of your country)
* ...
******
Maybe you want to go the safe way...then use the next larger diameter.
But mind, that you need more coil space for the thicker wire...and maybe the result is not that good as you assume, because the skin effect reduces effective/conductive wire area.
Klaus