Er....so ive made a small hallway in jkedit now how do i make a big sector from that small one...........any one please.....help??

Uhhhh... get JED ? Seriously, JKEdit isn't that great for much. If you want to use JED instead, you could just extrude the end surface of the wall, then extrude both sides of the new sector(keeping those with the new sector), then extrude the top of the new big sector. This could do it (however, do to JKEdit's interface, good luck getting the sector just how you want it.)
[edit]Sorry, meant JKEdit's interface. My bad...[/edit]

Or (still in Jed) you could ctrl+x (expand) the surface (extrude and do all that "make the next sector bigger" junk for you).
You'll likely have to rearrange some cleaves (probably by merging and recleaving how you want them, so get the Cleave2Vx plugin)

Call me crazy, but I just really can't figure out JKedit =\