Hello Tina,

Thank you for reaching out to us.

- (size of room, meals, number of people on reservation - Renting a room up to one day as service appointment is possible, meals can be extras and they can bring someone with them

- customers need to be able to pay a deposit, and then make payments for the retreat - Yes this is possible

It would be great if there is an automated reminder system as well, but not mandatory. - We have regular notifications and reminder notifications that are cron and they are set on your server by your hosting provider and then they are trigered in Amelia

Is this the right plugin for me? I've added a URL to an upcoming retreat that I would use your plugin on. - We can not know because the plugin cannot be used for multi-day bookings. It's currently limited to appointments and services with fixed durations that cannot go over into the next day. 
For example, your employee's working hours can be from 00:00 - 24:00 on all days, but if there's a service that lasts 3 hours, and someone would like to book that service at 23:00, the time slot will not be available, since Amelia can't go into the next day. 
This will be added at some point in the future, but unfortunately, at this time - if that what you're looking for to achieve then this is not possible.

We wish you all the best.