I have some acne on my back. Due to some pimples popping, I often get blood spots at various areas of my vest. These usually dry up and I am not able to remove them later. This has happened with almost everything that I wear. So, is my prayer valid in such clothes and how much dry blood is allowed on clothes for prayers to be valid? As far as I know, it is the size of a Dirham but how do I know the exact size of a Dirham?

If you wash the clothes and the color remains but no texture then it is pure.
Without washing permissible amount is combined or individual altogether less than the size of your thumb print