The suggested size of a DEVONthink database is measured in words, more than in bytes. You can see this in File > Database Properties for each database. DEVONthink indexes the text of files and stores that in an index for each database. When you open a database, its index is loaded into memory for faster operations. The more unique words in the indices, the more memory is required.
Generally speaking, a modern machine with 8 GB RAM can easily hold one million unique words and 100,000 items in the database.