In the BMD-300 Series Datasheet, there is requirement for a large board: 40x35mm, but our board is 30x30mm. Will this present any problem for RF performance? (antenna pattern)?
A smaller board will still work fine. The radiation patterns may be affected some, but not to the point of a severe decrease since your board isn't that much smaller. We just tested one board that was the size of a 2032 coin cell, and it had a couple more nulls and maybe a few dB lower in the received signal at our test kit.