With Simple Delivery anything over 61cms long is classed as bulky.   That's their rules.

The alternative is to list with free postage, factor that cost into your item price and buy your own postage outside of eBay.    But it is a rigmarole.

To get the buyer's full address and mark dispatched you have to click on Get prepaid label > Print label for the original Simple Delivery label.   It will remain unused and you can claim a refund for it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller

The option to change the tracking has been removed but you would be able to add the new number to a "not received" case if that were necessary.     Without tracking on ebay the payment may be held for 14 days.