Hi Alex,

is it possible to switch to Amazon S3 while we have several files already on our server or this needs to be setup before launching our site?

You can switch to S3 even if you already have files on your server. The system will upload these files to S3 each time cronjob executes.

Is there a way to reduce the size of photos at a minimum?

The only way to do this is to restrict user's upload behavior(eg: file size restriction, max amount of file upload per day, etc).

Is there advantage or disadvantage opting to store the files to S3 as offered within EasySocial?

Advantages should include low cost for S3 compared to webhosting providers, reliable security, availability. The only disadvantage I can find online is that if the site stores a large amount of huge video files, the cost of S3 may be steeper.