What you need is a pattern, and it will tell you how many yards of fleece to buy. I make pj pants pretty regularly (keep reusing the same pattern) and it takes about 3 yards for pants.
Here's a pattern I found that includes a pattern for both a robe and pajama pants. Most of the other patterns I could find were for either pants or a robe - but you will save money this way since it's all in one pattern envelope.
McCall's M6236
http://mccallpattern.mccall.com/m6236-products-12461.php?page_id=907
Scroll down on the webpage and you can see the back of the pattern envelope which shows the yardage you will need to sew each item, based on what size you want to sew.
Go to a fabric store to get your pattern, fabric and elastic waistband. I've found that the people working at the store are very helpful and will help you (if you ask!) with reading the yardage requirements to make sure you get the right amount of fabric.
Another pattern with both pants and robe is this one. It says Plus-Sized but I checked the envelope back and it's a gender-neutral pattern that is "plus-sized" for women and full size range for men.
Simplicity 5314
http://www.simplicity.com/p-2225-plus-size-sleepwear.aspx
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