The Arabic font size is fine, and, frankly, it's good as smaller
In my experience with the Arabic course (which I'm almost done with), the first time I saw the sentences, I too felt that the font was too small. I felt that it might be beneficial to increase the font size. Here's what I think now:
The font size that Duolingo uses for every sentence exercise is set for every language. Native Arabic speakers might have a heard time learning the Latin alphabet, and maybe on the Arabic > English forums, they say that as well. However, this font size trains you. It challenges you to read text like a normal Arabic speaker would. How would you be able to read in our font size for Arabic if you've had no experience with it?
I have absolutely no problem with the size now, even on mobile. I can only read the small font size because Duolingo trained me to.
It's the font that Duolingo has chosen, rather than the size, that is the problem. I can't see the difference between the diacritics. The diacritic with two dots looks identical to the diacritic with one dot. Duolingo's decision to go with the GeezaPro font has made an otherwise decent course unusable for me.
The thing is, the font doesn't matter. The font that the course uses is the default Arabic font that your device uses.
There are no default fonts on my device, which is a laptop running a web browser. If you use a laptop or desktop computer, and web browser, you can view the code that makes up the page. If you do that, you'll see that the font which Duolingo instructs the user to see is called "Geeza Pro". I compared this font to all the other Arabic fonts available to me and this is one of the more difficult ones to read, specifically because the two dots in the diacritic blend together. Many of the Arabic fonts on my computer are that way, but some are not. One called "KufiStandardGK" shows the two-dotted diacritic as discernibly different from the one-dotted diacritic. So, for me, it really is just a matter of Duo choosing a different font for me to see.
What device do you use? I'm pretty sure it's been proven that the Arabic course doesn't have a set Arabic font.
Thank you for opening this post. I never have recognized that the Arabic fond is changing even we change from Arial to Calibri. Yes, and indeed Duolingo is using on the PC the same fond I have in Word. Unfortunately Arial makes the Arabic fond very tight so that there is nearly no space between the letters. And this makes it also harder for reading these small letters.
But in one point I do not agree. Only if you got confident with the Arabic letters in reading and writing then it is ok to learn the language in their tiny written way. Let's say, this is what I have experienced. Now, after I have worked for some months to get used to these letters I recognize that it is easier to read words I already know in these tiny writings. But still if it comes to new words I need a larger fond for learning these word straight and without spelling mistakes. Actually it feels similar to kids in their first degree. They also get their first texts written in a larger size. :-))
That's an interesting perspective, but unlike learning kids, who can barely read, I'm sure we're all teenage to adults here. Of course, reading at first will be hard, and recognizing words will be the easiest method of reading. Fluently sounding out a word that you've never seen before is true mastery of a language.