pennyswings,
You can certainly use the standard 2.5 inch jelly roll width to make log cabin blocks. The block pattern will be there, but the size of your finished block will be different.
Using the 2.5 strips, each log of your block will end up 2 inches wide; using 1.5 inch strips, each log of the block would end up 1 inches wide. Depending on the size of the center square, if you use exactly the same number of logs, the 2.5 size will be about twice the 1.5 size. You would probably add fewer logs when using the wider strips, to keep your block size reasonable. If you simply follow the directions about light and dark placement, you will end up with the same block pattern.
I would make a sample block to see what size you end up liking for the quilt. From that sample block, you can figure out how much fabric you used, how many blocks you will need to make your quilt, etc.
Here is a website with standard measurements for quilts:
http://www.thequiltcenter.com/measure.htm . They give the usual size for a twin size (single bed) quilt as about 69 inches by 90 inches. In reality quilt sizes vary depending on pattern repeats, border sizes, whether you want the quilt to hang over the sides, etc.
If you make your quilt to the 69 x 90 dimension, you would want to order 2 yards of the 108-inch wide fabric for backing.
Mary