Hi

Only A4 size blank paper or A4 size fullscap paper is allowed for IGNOUs assignments.

I suggest fullscap paper then your writing won't go slant moreover you are supposed to leave few lines after each answer so that the examiner can leave comments there so any ruler lined A4 paper will do. From this year they have made submission of assignments online. Check all the instructions and read it carefully before submitting the assignments.

http://www.ignouhelp.in/instructions-for-ignou-assignments/

Hope this helps!