How to set subtitle size from plex ios app?

Hi,
I have just this question: how can i set subtitle size from plex ios app? I'm going to explain you the problem:
I can set sub size from web browser panel but the right size works only if I use the web player; i usually use plex from my iphone and send the video to chromecast (2). In this (second way) the sub size is setted by default very small and it's very hard to read it from a little distance. Probably there is something i have missed, thanks you all for your help!