Handkerchief size? We've been told various sizes. One person insisted that the accurate size was 20"x20" or 24"x24" for bandanna types. Ours are not based on an actual example we have. To be honest, the size varies a bit depending on how many Vicki can best cut out of the width of the cloth and not waste material. She also folds over the edges a couple of times and sews them, so a little width is lost there. She mainly came up with the 12x12/ 14x14 finished size; small enough to fit in a pocket, economical enough to have enough to give away to as many soldiers as we can. Somehow I don't think a Civil War soldier back in the 1860s would have said "hey, this isn't the right size ya'know" if someone was handing how the type Vicki makes at a hospital or encampment.
Cloth resource suggestion: This year, Vicki has been visiting the Good Will/ Salvation Army stores near us and looking for cloth to use for handkerchiefs, comfort bags, aprons etc. She has found some good quality sheets made of cotton, both regular and a few fleece-type that are usable for the handkerchiefs. So in the spirit of the USCC, she buys them, washes them and cuts them up to be used in her sewing projects to supplement the cloth she buys at the fabric store.