I think that you're probably out of luck then.
To the best of my knowledge, the software within 'standalone' printers can't handle 'soft fonts' (where the font size can be fully dictated by the user). Each font size is a separate file within the software, meaning that (for example) there's a file for 8pt and one for 10pt but not one for 8.5pt.
Sorry!
Perhaps it might be better to look for a label printer that prints from a PC? The Brother QL-600B Labelling Machine (£59.99, from both Argos and PC World) might meet your needs. See the video here to see what it can do:
https://www.argos.co.uk/product/7659746
(I'm getting tempted to buy one myself now!)