how to resize main window

This has been discussed numerous times, see the links below:
https://answers.launchpad.net/unity-tweak-tool/+question/224512
https://bugs.launchpad.net/unity-tweak-tool/+bug/1173678
The second link is basically your issues and we've explained why resizing won't help you at all.

Hmm ok but how can I access the settings which aren't display on the small display of a netbook ? I don't understand why it is unnecessary to add the possibility to resize the window, so that UTT can used on smaller displays with a smaller resolution.

Resizing won't help you with your problem.
The elements in UTT (and in Gnome System Setting and in elementary's Switchboard and in .... you get the point) are aligned in certain grid that does NOT changed with the windows size like for example Nautilus does (you know when you resize Nautilus window how the folders get rearranged).
Here is what will happen with resizing UTT in your case:
Before resizing: http://i.imgur.com/K1Ar5Ep.png
After resizing: http://i.imgur.com/8CWsn7n.png
As you can see resizing won't solve your issues, all that will do is create useless white space in UTT window.
Having said that we are indeed trying to come up with a clever way of fixing that, but so far we'vve come with nothing except detecing if the machine resolution is a tiny one and if that's so making UTT have rearranged design for it. That however is not a priority to fix as it requires large amaount of GTK and Glade work and having a ntebook layout will mean redesigning certain panes of UTT, which might result in the looking really odd on a netbook or worse - some of them might feature way too many widgets to be squezed on a netbook screen, which means thsi solution won't help at all.
If you have any quggestion how can we solve this sugegstions are more than welcome. Write them down here or jump on #freyja-dev Freenode IRC.
Cheers! =)

Ubuntu actually allows you to access the whole contents of a window larger than the screen: just press and hold the Alt key and then drag the offending window from anywhere. This is a super-handy feature that I miss in pther operating systems.

Yeah, moving it like that is a workaround.
Having said that in some version of UTT (we are not sure if it will be sooner or later) we will have no gtknotebooks (tabs) in the UI and when everything is panes only some panes will have scrollbars instead on notebooks instead of being longer than the display height.
And by someday we should also have automatic smart resizeable pane size (every pane is the perfect size, no whitespace of panes who has less widgets) with animation (like Gnome System Settings does it).
Those two should address all resizing issues for good.