13" is okay for matlab, you could probably get away with 11" but I think it would be painful.
If you want to run the CAD software like Solidworks a MBP 13" might be the better choice (256GB + 8GB RAM) comes out to cheaper than the MBA 13" equiv with upgraded proc. And could probably run it reasonably well-ish (better than MBA). You usually don't need to do a lot of model analysis in undergrad but sometimes those CAD programs can be a system hog. Especially if you get to complicated model assemblies.
If you don't want to run the CAD software the MBA 13" should be okay it just might take longer to boot up matlab but with the programs you do processing power shouldnt make that much of a difference. If you can hook up an external monitor an 11" might be fine as well.
Heres someones experience with an air and solidworks: http://solidworksonamac.com/using-solidworks-on-a-macbook-ai...