I can't activate the Nether Portal

[NaporZ 0]

So i have been playing on a public survival server, me and my friend, we built our base and then made more stuff bla bla then we got to the point we have to make a Nether portal, so i built the nether portal 6 blocks high and 4 blocks wide, and i am pretty sure the corners don't matter, and when i light it with a flint and steel, it doesn't do anything, it just lets the fire go off, i even tried removing blocks around the portal if that was the case, and also it was underground, under our base because we didn't want our portal public and everyone using it, im not sure if that's the case but i think that's why i can't light it - it's underground. heres a link to a screenshot also:

 

file:///C:/Users/Eriks/AppData/Roaming/.technic/modpacks/tekkitmain/screenshots/2014-06-15_06.41.50.png

[Kotja 5]

Nether portal is 5*4, bigger portals (up to 23*23) were implemented in 1.7.2 (tekkit is 1.6.4)

You will need 14 obsidian blocks (10 without corners)

[Niverton 11]

Also, we can't access files on your computer, the link to your screenshot is just the path to it on your PC. You need to host it on services like Dropbox, Imgur or Imageshack so others can view it online