What were the sizes of British infantry regiments during the Napoleonic Wars? Also were regiments one line or did they split into multiple lines during a line battle?

British regiments (we should, strictly speaking, refer to battalions really) were usually between 500-800 strong in the field. When battalions fell below 500, as occurred quite often in the long Peninsula War, they were either withdrawn and sent back to the UK, or bolstered with reinforcements (from the Regiment's other battalions).

The Guards were often larger, and at Waterloo, all 4 Guards battalions numbered 900-1,100 men, but this was extremely unusual.