I am considering making a free offline client for Wikipedia as a first app and I note that PD does not allow in app downloads so I would have to include the content in the download.
Is there a max size for apps?
Is there a facility to download a different variant of the app from the catalog based on, for example, the language selected?
Anything else I should know before undertaking this?
As of 2 July 2023, the size of the current version of all articles compressed is about 22.14 GB without media.
I think maybe having some kind of specific niche might be a way to keep it under control size-wise. The Playdate is only 4 Gigs and, while I like the creativity of the idea, many people would not be willing to dedicate their whole machine to one application.
There's an idea there, but just think about the audience and the hardware limitations.
Yeah of course the whole of wikipedia would be silly. There are ways to get a download of only the most popular articles (and some other filters I haven't played around with).
Due to the power law distibution (related to the "long tail" effect), I believe an app would be useful if it only included, e.g. the top million articles, with images and other assets stripped. I believe I could get that down to a manageable size. Probably as you say it would need to be less than 500mb for people to consider downloading it. Obviously there is a tradeoff here between completeness and portability.
I will repeat my earlier questions because I think they are still relevant:
Is there a max size for apps?
Is there a facility to download a different variant of the app from the catalog based on, for example, the language selected?
thanks for your time
Did some more research, the encyclopedia brittanica in the 1970s contained roughly 70k articles (over 32 volumes!). If we take the reported average wiki page (~40k) and muliply them we get 2.8gb. Probably it wil be possible to strip some cruft out of those wikipedia dumps and get that down a lot more. Also just not including really long articles etc. I am still optimistic that this is feasible.
edit: I am told that zip can compress html by 20-30%. That's gotta help too.
I think you're optimizing prematurely. Don't worry about these things until you've proven the concept.
First, I recommend making a reader app that works with a single page 
I imagine it will be tricky to render the article and display images. And then there's enabling hyperlinks.
One idea would be to make a versatile viewer that the user can add their own content to. There are already readers for static text files.
Your questions:
not sure about maximum size but if I recall correctly it's a magnitude smaller than any of the sizes quoted in this thread
there is no way to download variations
Also, apps/games need to go through a submission process for Catalog so there's no guarantee of acceptance.
One idea would be to make a versatile viewer that the user can add their own content to. There are already readers for static text files.
Can you give an example of a reader? (actually I found some good examples)
A quick search brought up your project here: Strategies for Playdate by gingerbeardman
I hadn't considered USB as a way to add content, so that is an idea.
How do I find out #1? I need this info for this project and others.
My app Strategies is more if a flash card thing. Small amounts of content presented as cards.
For a book reader check out PlayBook - A smooth scrolling, delightfully comfortable ebook reader for the Playdate! - #6 by Indiana-Jonas
Answer to max filesize "Panic recommends a file size of around 20 to 40 MB for an average Playdate game." Frequently Asked Questions - Playdate Community Wiki
If you do go down the USB route you might want to time how long it takes to transfer a really large file to the Playdate. I think even 1GB would take a very long time.