Thanks for your question!
I don’t have any specific dimensions, and like you my searching online has only given me the length and weight of dugongs. With this limited information, it is absolutely fine for your team to make an estimate/assumption about the width and height of dugongs for the purposes of your tank. When you are unable to find specific information it is absolutely okay to make informed assumptions (in this case, based on the weight, length and knowledge of the shape that dugongs are), as long as you highlight where you’ve made assumptions in your report.
I would also encourage you to think about hw dugongs do come in a variety of sizes, so making sure that your design can accommodate for these variations too.
I hope this helps, please let me know if you have any other questions!