How much transient data can I allocate?
Not much. The value is platform-dependent, but it's usually around 220 bytes. Transient space is shared among all applets, so you need to be very conservative. Unfortunately, there's no API to get the amount of transient space that the platform supports.
As a consequence, try to stick to the CLEAR_ON_DESELECT type, because since this type of data is meaningless when the applet is deselected, the platform should be able to reuse the space for the current applet.