A long time ago, I read a quote that roughly said that "the plaid should be big enough for a man and his beloved to sit comfortably and have a picnic".

You will have to carry it all day, so I would go for the biggest size that you can comfortably carry around on your shoulder for any length of time. If you work out the length/weight and carry that weight around in a pillowslip, on your shoulder for a couple of hours, you will probably get a good indication of how well it will work.

The other thing is that if it turns out to be too long it can always be shortened, but not the other way round.

Would be nice to see pictures when it is done.

Regards

Chas