At the top of the mast
there will be a little "wheel" called a "sheave"
set into the mast. Near the deck
there will be a sort of hinge on the mast where the boom attaches to it. That hinge is called the "goose neck".
You need to measure from the bottom of the sheave to the gooseneck, and then from the goose neck to the end of the boom.
The corner of the sail at the goose neck is called the "tack". The corner out at the end of the boom is called the "clew" and the corner at the top is called the "head"
The distance from the sheave to the gooseneck less about 6 inches will be the length of the leading edge of your mainsail
called the "luff". The distance from the goose neck to the end of your boom, less about 6 inches, will be the length of the bottom edge of your sail, called the "foot"
Look at this:
You can see two things: 1) there is a curve to the trailing edge of the sail. That edge is called the "leech" The area of the sail that lies outside the straight line from clew to head
is called the "roach" 2) the mast leans slightly towards the back of the boat
. That lean is called "rake" and it is measure in degrees from the vertical.
To "lay out the sail" (when you are making it) and to buy a second hand one, you need to know the degree of rake. Hoist a plumb line up on the halyard
and let it swing free. The distance it comes aft (behind) of the mast at the level of the boom tells you how much the luff needs to "lean back" from the foot.
With this little sail you can no doubt find a room big enuff to lay out these dimensions on the floor using green masking tape to represent the edges of the sail.
Now you'll know what you are looking for :-)!
You can get much more scientific than that, but I dare say that at your stage of development as a sailor, that'll do.
If you can find a secondhand sail that is just about right (as to dimensions) we'll tell you how to modify it so you can use it.
All the best