Limit to Expression Size in Query ?

I have a very complex expression that I need to code in a query.  It uses 6

fields from the table to calculate a value that I will display for now.  
Eventually that value will be compared to another field as a criteria.

1)  Is there a limit to the size of the query?

2) I wrote a VB function that provides the result I need - Is there a way
to 
use that function in the query?

3)  Is there some other approach that might work for this?

Thanks for any help you can provide