'Forest' can mean two things in England. It can refer to an area covered by trees, or can refer to an area once treated as a large hunting estate, usually formerly belonging to the royal family. Many of these 'royal forests' were never wholly covered by trees, and are not today.

The largest tree-covered area in England is Kielder Forest. (The English -Scotish the border runs through the forest, but the part in England is still bigger than any other English tree-covered area).

There is some dispute over the size of the old royal forests as some were abolised in the 14th century and it is not clear now exactly where their borders were. The largest is probably the New Forest, although it is possible that the Forest of Pickering or Sherwood Forest may have been a big larger. The New Forest still exists as a distinct geographical area, but only about half its area is covered by trees, the rest being heathland and rough pasture.

Possibly Kielder Forest in Northumberland

Yes I agree with the other two answers. The Savernake Forest in Wiltshire near Marlborough is pretty large too. So is the Forest of Dean in Gloucestershire stretching towards the Welsh border www.visitforestofdean.co.uk

and the other sizeable forest is the Mortimer Forest straggling the Shropshire/Herefordshire border near Ludlow.

I know these 3 forests intimately. They are gorgeous

The Wark forest in Northumbria. It is a big conifer forest largely planted after WW2.