The primary goals of the rifle weapon type are accuracy and range.
To accomplish this, these weapons are usually designed for two-handed use for stability during aiming. Most rifles have a long, slender aspect ratio to give them a natural line to aim along - for those weapons that have no targeting enhancements. Otherwise, there are generally no severe size constraints for rifle design - other than sheer mass that a being can heft - as they are not usually meant to be concealed.
Rifles are the only type of weapon that have the necessary precision for sharp shooters to perform their art.