I have got to make a robot that can calculate the volume of a box (only width will be given) and I have no idea what sensors to use.
Need suggestions from the pros.help me. 
X-ray computed tomography, or, if the stuff the box is made of is very dense, gamma ray computed tomography.
If I were you I'd not think about hardware solutions right now, but brainstorm (even if you're a group of 1) and think about some ideas, no matter how crazy. Then see what kind of sensors might exist to sense the things your ideas tell you need sensing to give you the input to some calculations.
Have a look at this process. Your background research here, to me, would be looking eg at what kinds of sensors exist, not thinking about which ones you should use.
guys, the robot also has to drop a few hard boiled eggs into the box ,which we have accomplished.but we are at a total loss when it comes to finding the volume and I was hoping that you would suggest some things we could work off of.
Put a scale under the box, have the robot fill it with water, and record the difference in weight. 1 gram = 1 cubic centimeter of water.
we could work off of
We?- so you're a team? Get some flip chart paper and sit around and throw crazy ideas at each other... capture them no matter how whacky they seem.
If you're doing the egg thing, perhaps you already have arms, which could "embrace" the box much like a caliper. Then the geometry of the arms (angles, lengths) could be used to calculate the dimensions.
Also related to the egg thing, since the box is thus open at the top, you could have a distance sensor attached to the dropping mechanism which could measure the depth, assuming the sensor was flush with the top of the box. Have you Googled distance sensors at all?- infra red and ultra sound.
Is the box in the middle of a table or can it be next to a wall? If next to a wall, then one of those sensors could ping the distance to the wall, move a long a bit and ping the box, subtract the two. You could do the height that way if the box has a lid.
Go dig around on sites like adafruit and Sparkfun to look at measuring devices.
I suspect your instructors have in mind that this project is as much about you working through a design process as implementing a perfect answer. Start thinking...