Howto determine GPU tile size ?

Yesterday i helped someone with a GPU render bug with the adaptive sampler branch.
I tested if my CPU octo core, had the same problems, it didnt.
But then i also checked if my ‘poor’ geforce 630M would show the render error, it didnt either.

A bit to my surprice my GPU with adaptice sampling was faster, then the CPU in this simple scene.
As i used his blend file, his tile size was 160x160
Impressed by the faster rendering, i opened some of my own blend files and tried to render them to with GPU.
However then my system crashed with a bleu sceen twice, and several other times The GPU driver blablaba technical connection lost, and popups that it was reconnected again by windows… but then blender needed restart (or pc)

So now i wonder i guess when i overload the GPU my system crashes, or blender looses the driver connection.
How can i determine what is a proper tile size, without doing a trial and error of bleu screen experience ??

windows 7, intel i7 2.3 Ghz, 24GB ram, Geforce GT 630M (i cannt update this graphics car).

630M - that would be mobile, laptop that is. Gets pretty hot under the load and if something goes wrong…
GPU driver blabla under the Win can be cured by changing/adding one line in windows registry. In general this is precaution saying that windows finds suspicious that GPU is not available for time more than some set parameter and there can be something wrong going on. They did not know about Cycles at that time…
Look in the Tech Help threads - exact line is mentioned somewhere there. MS has page http://support.microsoft.com/kb/2665946
More on registry changes here - http://msdn.microsoft.com/en-us/Library/Windows/Hardware/ff569918(v=vs.85).aspx
Funny thing is - you probably can add external GPU using some PCIE magic. https://www.youtube.com/watch?v=ktUw2VTFyX4

intersting link about external adapter…
however i have the asus k95v no external PCIE slots, “only”, 2 usb3 and 2 usb2 slots, and a hdmi slot
But i did found this usb3 to pciE adapter https://www.youtube.com/watch?v=oq2wQB8SuQs i wonder if that would realy work

Considering all heavy load is done on GPU and thus you do not really need stellar transfer speeds it might work. Only thing you probably would need to do in addition is this mod to the videocard itself judging by hardware seen in your link :D.

Edit: you could have this bluetooth plug mentioned, internally, i kind of see it here https://www.youtube.com/watch?feature=player_detailpage&v=Of56nS8nOQQ#t=1065. If such deep incision is an option of course.

I have used some toys like this on a laptop like machine (atom processor board) Its a mixed results thing. It did work and did work decently BUT there is some bottle neck issues. Still its worth checking out. If you get one let me know how it handles.

But i dont hace pcie…and the link i posted turned out not to be usb3