I'm growing 2 plants in a wardrobe, they are small at the moment but how big should I let them get?

That'll be fine just remember to switch to 12/12 when they are about half the size you want them to be at the end, they will double and maybe tripple in size once you switch.

I'm going to start a cupboard grow in the next 2 days (just came into some money for a light)

The cupboard is 1mx1/2mx1m tall. 1m tall Isn't a lot, but would it be OK to grow some quite short plants? Or I could just crop them to keep the height down.

Also, If I use a computer fan, roughly 10x10cm. Would that be enough to filter the air?

Any advice would be good.

Yep you can grow it almost anywhere. I wouldn't use anything like a HID though, unless it's cooled.

You could SCRoG, which is putting a mesh wire or string over the top, so only the end product will pop through. This is probably the best option for small grows.

The fan should be fine in a space that small.

To answer that question you need to tell us
A) how tall is the cab?
B) what size light are you going to flower under... don't want them too tall with low power hps... and you'll want to scrog if using CFL's
C) what strain are you growing... Indica/ sativa? some stretch more than others

I grew for years in a 400w fridge... 2' x 2' x 3' tall

Hi Shaggy, here goes ...

It's just over 5ft high, almost 2ft in dept and 4ft across.

I have a 600 watt HPS and I'm growing white rino.

with a 600 you don't need to worry about penetration... i would lst them untill they fill your area but keep them below 2'.. then flower them...