Hi Andrea
I just use Format > Resize Image and keep the 300dpi option locked and then change the size to 12 x12 from 8x8.
This is supposed to keep the resolution at 300dpi for the whole layout but even if it doenst at worse cas ethe resolution of the 12 x12 wil drop to 200dpi which is still perfectly adequate for printing.
The best idea is to try it for yourself. Create one layout at 12x12 and then a layout at 8x8 and convert it to 12 x12 - print both and see if there are any differences.
Let me know how it goes
