I'd add a 'make sure you take a copy of the DTSX file before editing' warning to this. Do it wrong and it will no longer open in SSDT. Of course, if your packages are source-controlled, this is taken care of.
For those who are interested in the structure of SSIS files and want to dig deeper, I'd recommend looking at the XML in a dedicated XML viewer/editor. Here is an example – makes it easier to see what is going on and navigate the entire thing.
Changing the maximum size for many columns is tedious in SSDT, though I would warn against simply maxing the default in all cases. Doing so is much the same as creating a VARCHAR(500) column to store data which should not exceed 25 characters in length – some implicit data validation is lost by doing so. There may also be a performance penalty (I'm not sure about this). I tend to define SSIS maximum sizes to be the same as the columns they are being imported to.