How do i get large option on pootage

[nutberry6]

Need large option on postage but all it gives me a medium which is not big enough.  It's not heavy it's light but the parcel is bigger than the medium dimensions 

[**peaseblossom**]

The "Oversize" option, where you choose your own postage, is only available in some categories.    Change category or sub-category until you get the option at the bottom of the pop-up (after cilcking on the box under the heading Package Size). 

@nutberry6