Length plus twice the diameter at a maximum of 104cm
Greatest dimension no more than 90cm
Up to and including 45cm in length, 8cm in diameter and 2kg in weight can be sent as a Small Parcel
So you can send a roll or tube up to 90cm in length. but it wont go as a small parcel as its length is over 45cm. The price checker bit of their website says for tubes over 45cm and under 90cm, then they go as medium parcels.
Cheaper than they used to be before a couple of weeks ago…
BUT, wow, extracting info from the website is painful, and they have so many categories they’re just shooting themselves in the foot. I can’t see this as going well at all for royal mail, and especially all the post offices that are left.