Execute clinfo.ijm from here.
It will print out information about installed GPU devices.
From this log answer these questions
- What GPU is used per default?
- Home much memory does it have available?
- How large can a single image be when being processed in the GPU?
- Assume your image stack is 1024x1024 pixels large and of type 32-bit float, how many planes can it have so that it still fits in GPU memory?