I just have a few quick questions..

[Magnus]

Making a tunic is very easy, no design needed. Just measure the span on your elbows this it your width

Wrong. Go by that measurement and your tunic isn't going to be wide enough. Most bolts of fabric come in 60" widths...you will need minimum half that width to make a functional tunic. When it is bloused at the waist and hanging by your thighs, a tunic with anything less than that will hamper your ability to move. Have you actually made your own tunic Matt?

Long sleeve tunics are acceptable as well, though I am not sure of any specific design for that.

Wrong. If the OP is going for mid-1st century, it will be short sleeves.

Linen and wool are both historically accurate. Though wool most probably much more predominant.

The predominance would have been based on the location of the legionary, as well as the weight of the fabric, linen or wool. (think England vs the Med).