what is the best mold to create large silicone sculptures?
You are saying that your sculpture will be made of silicone polymer?
you need to be more specific;
What is your objective?
How long do you want the casting to exist?
There are several different types of silicone and not all have longevity after they are cured.
In the land of 2-part RTV silicones, Platinum-based silicones provide the best archive-ability.
I have cast that type into molds made of Shell Shock brushable plastic which is available from Smooth-On.
Platinum silicones can be cast in plaster molds. However beware that platinum silicone curing can be inhibited by some materials with which your mold may have been in contact with. For example sulphur in some clay formulas. So if you want to model a figure, do so in one of Chavant's sulphur free plasticines.
Further, you need to use a separator or release agent. Dental separator solutions work well.
If you are to begin casting, and you are planning to just cast a skin, then you will need a thixo so that the material does not drip.
silicon moulds can be used with release agents to cast silicon duplicats