> firebird don't resize fields to smaller value even it is empty. How to solve this problem?

Add new field, copy data, drop old field, rename new field to old name.

> I need one more tip from firebird masters, is it smarter for me to use varchar fields with 1000 - 3000 length or text blob fields in case of performance and better work?

Try to read this: http://ibase.ru/devinfo/charvar.htm

SY, SD.