I never would have expected my students to spontaneously get out rulers and start arguing among each other -- using mathematical language and reasoning of course -- but since they did I am online looking quickly for resources to help me show them how volume is calculated and some practice pages for those who want more.
Volume Cubes - Easy
Volume Cubes - Harder
Volume Cubes - Hardest
Montessori has a material for building figures and calculating their volume using small wooden cubes, so these sheets will be easy to implement in the classroom.
Small Wooden Cubes for Volume with Box
Or, if you are going to teach the traditional formula, try
Volume of a Rectangular Prism (l x w x h)
And, if you're the type who likes to make your own worksheet, try WorksheetWorks.com - Calculating Volume. This website allows you to customize the problems on the page (simple figures or complex figures, English/Imperial units or metric units or a combination of both, and your desired paper size). Then it creates a free worksheet for you with reproduction rights allowed for a home or classroom for up to fifty students. And they give you an answer key!