Theoretically, up to 32gb. This is a result of using 32 bit “addresses” (4 gb) with 8 byte alignment.
In practice, I would not use cSuneido (the Windows exe) for more than 1 or 2 gb because it is a 32 bit program and therefore can only memory map that much address space. It will map segments to get around this but it will be slower.
For larger databases I would recommend running client-server using jSuneido (the Java version) as the database server. Running on a 64 bit JRE this allows a much bigger address space.
jSuneido also has a new database engine which is faster and more crash proof.