39-40 kilobytes. the game code (incl music) was about 31k and the graphics were about 8k.
The entire level data for 1-1 is just this (level layout first, then enemy data)
L_GroundArea6:
.db $50, $21
.db $07, $81, $47, $24, $57, $00, $63, $01, $77, $01
.db $c9, $71, $68, $f2, $e7, $73, $97, $fb, $06, $83
.db $5c, $01, $d7, $22, $e7, $00, $03, $a7, $6c, $02
.db $b3, $22, $e3, $01, $e7, $07, $47, $a0, $57, $06
.db $a7, $01, $d3, $00, $d7, $01, $07, $81, $67, $20
.db $93, $22, $03, $a3, $1c, $61, $17, $21, $6f, $33
.db $c7, $63, $d8, $62, $e9, $61, $fa, $60, $4f, $b3
.db $87, $63, $9c, $01, $b7, $63, $c8, $62, $d9, $61
.db $ea, $60, $39, $f1, $87, $21, $a7, $01, $b7, $20
.db $39, $f1, $5f, $38, $6d, $c1, $af, $26
.db $fd
E_GroundArea6:
.db $1e, $c2, $00, $6b, $06, $8b, $86, $63, $b7, $0f, $05
.db $03, $06, $23, $06, $4b, $b7, $bb, $00, $5b, $b7
.db $fb, $37, $3b, $b7, $0f, $0b, $1b, $37
.db $ff