It shouldn't be related to size. What is the wiki, and what is the page you are using to search for it? Please give links not just names.
Where have you tried to search it on? Saying "FandomMobile" doesn't explain it.
Is this the Fandom App? If so, only selected wikis are included.
FandomMobile is not the app, it's a browser version for mobile.
Oh... So how can my wiki become a selected wiki?
Is there some process it has to go through?
You can contact Fandom Staff and suggest your wiki but I don't think there is really a chance. Selected wikis are, well, selected. Wikis with a lot of high quality content.
FANDOM has been asked several times what their criteria is for the FANDOM app and they will not give it out. A long time ago they hinted at some factors, but would never confirm.
As the others have said, you can ask Fandom since the criteria aren't public. However, if I had to guess, they will probably refuse your wiki; at least for now. Generally, wikis need to be well developed and have a very large audience. From the looks of things, your wiki is relatively new.
What do you think?