Nether portal built in nether doesn't light up with flint and steel

Hey,

My son crafted a 21*28 (21 high and 28 wide) portal in the nether using obsidian blocks in creative mode and tried to light it up using flint and steel but it wouldn't light up. We were wondering why. He made another 12*17 one (12 high and 17 wide) right next to it thereafter and that lit up fine.