Depending on the final size you want the quilt to be ... I would not recommend 1 1/2" squares for a bed quilt (I speak from experience!!!) you will end up hating the thought of working on it!
I made my first Trip quilt using 5" sq and it was perfect then I thought I'd jazz it up a bit and made my daughter one for a queen size bed out of shades of purple ... using 3 inch squares - I was ready to kill her and myself byt the time I got it done ... and to this day I have no desire to do another purple quilt (and it's been 5 years!!!!)
Just my thoughts ... and mine was the Eleanor Burns verson ... but I had the pattern using squares - not rectangles.