The following might not even be possible. If not, tell me straight up.
In EF4 (without the CTP) you can turn off code generation to make your own poco classes. You then mock them up in the designer so that they match your actual classes, then choose the “Generate DB from Model” option.
The problem is that I see no way to alter what it generates. Sure, you can edit the .sql file it generates, but the designer sets all strings to nvarchar(max). The properties pane for these are all greyed out. Is there a way to set these yourself within the designer?