big, deep, large
comfortable, comfy (informal)
plush
old
overstuffed
leather, upholstered

be seated in, be slumped in, be sprawled in, sit in
collapse into, flop into, throw yourself into
curl up in, ease yourself into, settle into
He eased himself into the big armchair.

Armchair is used with these nouns:
fan, traveller, viewer, warrior