Nanaky, I've tried the above control and it is not working. Throughout the entire range option, the font remains tiny. I have purchased many ebooks in the past and none of them have behaved like this one purchased from Booktopia.
Then its the formation of the eBook. Sometimes a eBook is create that the reader can not change the font size. Aks your reseller if exist a file how you can change the Font Size.
Did you ever figure this out? I'm having exactly the same issue on my desktop but not on my laptop.