Since you can't change the size of a megaprim to anything
you want, you have to find one with a size close enough
to what you want. There are a few tricks you can use, to
increase your options.
Although you cannot change the X, Y, or Z size of a
megaprim, you still can edit all the other parameters.
You can use "path-cut" and "dimple" to reduce the final
size of the prim. You can make a prim that is half the
thickness and/or half the width of the original megaprim,
and a wide range of heights less than the original
height. This gives you more sizes to choose from, but you
still can't have just any size you want.
Since the possible sizes
are all based on the unchangable X, Y, and Z parameters,
it's best to find megaprims with as many different X, Y,
and Z parameters as possible. While you may find
megaprims with all sorts of final sizes, they are really
based on a much smaller number of base megaprims. So
finding different base megaprims is what is important.
(Having hundreds of megaprims of different sizes in your
inventory is therefore just unnecessary clutter!)
Here is a screenshot of the edit window, showing both
the "general" tab, with the prim's name, and the
"object" tab, with its parameters. (Click to see a
larger image.) The prim's dimensions in its name are
circled in red, since they aren't as important. The X,
Y, and Z parameters of the original base prim are
circled in green, because they are what you really need
to look for, if you want to increase your possibilities.