This simple little guide can allow you to easily check if a letter/large letter will pass through the devilish hands of the royal mail postal service. The exact Size limitations can be found here: https://business.help.royalmail.com/app/answers/detail/a_id/89/~/size-and-weight-formats-for-uk-mail
As my build plate only had a size of 20cm x 20cm i was forced to cut some of the sizes short as otherwise it wouldn't fit onto me bed. So, perhaps just use this is a thickness sizing guide, or for packages smaller in length.
Probably best t.o print with 100% infill to keep it's strength, that way you can be sire that the thickness of your parcel does fir, and that it inst just bending the sizing guide.