Latex mask sculpture into a silicone mask conversion???

It is possible to shrink a sculpture, but the process can be expensive, time-consuming, and hard to control.  There are casting mediums that shrink based on additives.  This page can help a bit:  https://www.smooth-on.com/support/faq/155/

For a silicone mask to fit properly it needs to be created at the proper scale on the right core so everything lines up properly. The sculpt needs to be designed based on the core, not the other way around.  The sculpt is dependant on the core to control how thick the silicone will be in specific areas, which determines how the mask will fit, stretch, squash, and perform.

Adapting a sculpt designed for a latex mask that was not created on a core designed for a silicone mask is a bit like putting a square peg in a round hole.  If you try really hard you might be able to get it to work, but you may be unhappy with the results.

Using a base form that is not intended as a silicone mask core (like the 2 suggestions you listed above) could possibly work out ok(ish), but the resulting maks would not have the fit or performance of a mask created on a core designed for silicone masks.

There really are no rough head dimensions for something like this, since it all depends 100% on your core or lifecast and how snug you want the mask to fit.   You can start with a life cast and then shave down or smooth out features to create the right core.  Core design for silicone masks, and how they control the fit around the eyes/mouth, is a complex subject that is a bit too difficult to cover in text alone.   A good example would be the mouth, as you mentioned.  It needs to be sculpted in a slightly open position and the core needs to allow the clay to wrap inside and create that cupping effect, but describing it without a reference core would be difficult.

One option would be to design your own core and then take your existing sculpture and carefully cut it up and re-apply it to the new core while being mindful of how thick the clay is in key areas.  This will require a lot of re-sculpting and cleanup, but it's possible.

Sorry I don't have better news for you.  I'd love to see how your mask turns out though!

/Chris

I don't know of any off-the-shelf cores for silicone masks.  A product like that would be amazing, and I know I'd buy a couple for sure!

Most folks just have to make their own.  Getting a good core is also an iterative process, where you'll make adjustments and create new cores as you and other people try on the masks they produce and find out what works and what doesn't.  The companies that make silicone masks have all put many years of work and a lot of resources into developing their cores. 

/Chris