How large will UseBB databases be compared to some other popular forum software such as phpbb? How much space would for example a 1000 posts take in the database?- Well, I guess this will somewhat be the same size as phpBB or other forum packages. No HTML is stored in the database, only BBCode, so that can make a difference with BB software that parse BBCode before storing it. The size will now definitely be smaller as FULLTEXT isn't being used because the search engine is not ready yet.
How well will the search engines be able to spider UseBB? Phphbb had difficulties because of sessions (which could be removed for guests wihch weer search engines)- Each session gets its own session ID which is passed trough the URLs if the user agent doesn't accept the cookie which normally holds the ID. I am aware that this can be a problem to some search engines, as UseBB does exactly the same what phpBB does. A solution might be worked out later. I also tend to built in a feature which uses search engine friendly URLs instead of the .php?abc=1 like now.
How about support for international characters? (for example Japanese)- There is currently one problem with other charsets, that is that content added to the forum is viewed as a bunch of &3467; and similar. This should be fixed in UseBB 0.3