I would make it as big as possible!
I just built a 2x4 for my 2 boars and a 2x4 for my 3 girls. I wanted to make 2x5s. But I have the smallest bedroom in the house and after I measured out my room and rearranged furniture, I still could only manage 2x4s. It is the absolute minimum for the amount of guineas I have. They have bigger cages (two 2x6s) where they can go in the summer, in the daytime, in our shed, and also a nice big run which goes on the grass, and have floortime in the kitchen and the conservatory.
For two piggies, a 2x3 is the absolute minimum, a 2x3.5 is better, but a 2x4 is ideal, anything bigger is great! I would also base your decision on how much floortime they get. The less floortime they can have, the bigger the cage should be in my opinion!
Carefresh is a good choice, which is commonly used. I'm not so familiar with Aspen, but I think that is also. Have you considered fleece at all? Laura did this fantastic thread on the subject. It is my first time trying fleece, the boys have been on it since yesterday. View the thread via this link
http://www.guineapigcages.com/forum/...ect-study.html. What most members do is create a 'kitchen' area where they put bedding such as Carefresh, and this small area gets cleaned daily. This is separated from the fleece part of the cage, which is cleaned at least once a week. It just makes it easier as the piggies tend to do their toilets in the kitchen area and it also keeps food/hay off the fleece.
Good luck.