Sure the strike zone in fastpitch has dimensions. Some are stretched and shrunk by some umpires but the strike zone is still defined from rule book to rule book as being the same.
As a batter, you can help yourself tremendously by applying the strike zone "box" vision in your at-bat. The ability of a fastpitch hitter to expand and shrink the strike zone, according to the count, is a vital skill for a player to have.
Let the Count Decide the Zone
When you first enter the batter's box, the count is 0-0, no balls and no strikes. After the first pitch, and if you are still at the plate, the count will be either 0-1 or 1-0.
Now the strike zone "box" should begin to change for you. As I go on you'll better understand. If, after the second pitch you are still there batting, your count should be either 0-2, 2-0, or 1-1.
Behind in the Count
If the count is 0-2, your vision of the strike zone has made the "box" greater in size because the next pitch can strike you out if you don't swing at it and it's close.
Ahead in the Count
If the count was 2-0, then the strike zone has made your "box" much smaller and you should only swing at the pitch if it is thrown in your vision "box", your select spot. If the count was 1-1, then your "box" is back to being a normal size strike zone.
The key points of these examples are that if you have a 2-0 or 3-0 count on you, look at swinging only at a specific pitch you like in a specific small zone or "box". Perhaps only a fastball right down the middle.
If your count was 0-1 or 0-2, then the pitcher is in control of the situation and you must expand your vision of the strike zone so as not to be called out on strikes. Be prepared to swing at pitches you may not find as sweet as you prefer.