There are few basic options to set up and external monitor in XFCE including "expand to right/left" but that's not enough I want to set position exactly because my monitors have different size.
How do I do that, every other system/DE I used had advanced drag&drop monitor position setting.
You can use a program called ARandR. It is basically the perfect program for your needs.
Description:
ARandR is a visual front end for XRandR 1.2/1.3 (per display options), which
provides full control over positioning, saving and loading to/from shell
scripts and easy integration with other applications.
If not already done, you can install ARandR via the Software Center or sudo apt-get install arandr.
arandr, which pretty much does what you ask for. – s3lph Nov 30 '14 at 21:36