Juban is a type of underwear which you wear under your kimono, but is very important because it forms the fundamental part of your kimono. For formal kmonos, it is a custom to wear white juban, but other than that there is no particular rules.
Juban’s orginal role is to prevent the kimono from getting dirty by sweat, etc. Therefore, the size of juban is required to be nearly the same as the kimono, and does not show from the hem of the kimono. If you are wearing ready-made kimonos which are made by standard sizes, or kimonos made to fit your size, then you can reuse one juban for those kimonos.
However, if you’re wearing given kimonos or recycled kimonos, then you have to choose a juban that matches the sleeve width or sleeve length of the kimono you want to wear. To resolve this problem, jubans which sleeves are easily detachable are commonly seen recently.
Juban’s another role is to fix and maintain the shape of the collar. For that purpose, you attach haneri, and put in the erishin. As for the material of juban, silk was common in the past, but nowadays for common types, cotton is used for the corsage, and polyester is used for the collar and the sleeves to make it easier to wash.