It's waterproof (rain anyway, salt water or solvents might be an issue).
W x H x D is 160 x 180 x 6 mm, but side with buttons rises to about 8.5mm.
I use mine without a sleep cover. The other four people in the family with Sage models use a 3rd party sleep cover. A generic power bank is far better and cheaper than the official power cover.
Some complain about about battery life. It's certainly less than many 6″ ereaders, but fine, especially if front-light, Wi-Fi and BT are off.
Argos was the only local store that had them here and are now gone (1 purchase). Four were variously bought on Amazon or direct from Kobo.
If the Sage is too big, then get a Libra 2 or Clara HD and give up PDFs!
It's is trivial to install or un-install KOReader or patches on most Kobos as no Jailbreak is needed. I don't feel the need for either. Read about 26 ebooks since 25th December 2023 to 19th January 2024. Usually charges from PC while I'm fiddling with Calibre
Calibre lets you set title, author, collection, series and subtitle for metadata library browser in 'My Books' on Kobo Models.
The configuration file can be user edited and SideloadMode=true turns off marketing.
Calibre also manages reading status & position, highlights, annotations, bookmarks for Kobo Models.