RE: Stormtroopers (now Militarium tempestus Scions)
I'd say the average, if such a thing is possible with the variety in the Guard, regiment size for Infantry is probably around the 1000-1500 mark for combat personnel only. The regiment as a whole would be larger when taking in to account headquarters troops and other logistical and support elements but I'd feel safe with that 1k-1.5k figure.
A Stormtrooper regiment would be of comparable size but, as has been seen in numerous works of fiction, they will almost never function on the battlefield as a single company, let alone a regiment, and instead are attached piecemeal to other forces and armies that need their abilities and experience.
The notable exeption to this are probably the Cadian Kasrkin (who again have appear to have been replaced) who could be expected to fight in Company level formations quite easily.