I did some playing with the numbers and just decided to write a small spreadsheet to calculate the max pressure of a given bore size of a piston against different mould sizes, given a constant max cylinder pressure.
I am not a Gouda expert, but It looks like John has used about 2.7PSI on his Gouda. (80lbs on 6” wheel) in the past.
So, given his PSI, and your desire for a slightly bigger wheel (18cm~7inches.), you will need a bit more downforce than his 80lbs.
If you look at the chart, at 150PSI, you can get those kinds of pressures with the 25mm bore.
Looks like the most PSI your rig can deliver to a 7in wheel given 150PSI and a 20mm bore is 1.9PSI
As you can see by the chart, a 25MM bore will be able to deliver 2.97PSI, more than enough for your sized wheel.
I have included the actual spreadsheed if anyone wants to tweak it, or check my math for errors. Trust me, I make em.
<Clarification>, the pinkish area in my graph is the PSI being applied to the cheese.