It seems when I try to fire any projectile weapon, I get a notification that neither me or my squad has any weapon that is in effective range. Where exactly should I be in relation to a create or a bandit, if for example, I have a sniper rifle? And also, does the range criteria in the weapon stats is equal to squares, or something else?
Rules -> Technology -> Personal Weapons. That will give you a list of all weapons in the Combine and their minimum and maximum ranges. As long as your targeted enemies are within those two ranges when you are attacking with your weapon you will be able to hit your enemy.
For example, a Nightstinger has a minimum range of 10 and maximum range of 15. This means if you are at a range of 9 or below you will be out of the effective range for the weapon and automatically miss. The same thing occurs when you are 16+ away from your target.
If your squad and yourself have different weapons you will have to check to see if your weapons have any ranges in common. For example, if your squad has bowcasters and you have an A280 you will have to fire from a range of 7 in order for both you and your squad to hit the target. Sniper rifles are difficult because both you and your squad has to be using sniper rifles because they do not have any range overlap with other weapons. Except for the Nightstinger and A280, of course.
The Scanner page shows your range to target, and the Combat page shows your battle range as well as warns you if you don't have any weapons in range. And the min/max ranges are on each individual weapons page, as Dol pointed out.