I just bought RB1100AH which has MicroSD supported. I would like to give a try this RB for a web proxy for at least 100 users.
My questions are: (1). Is RB1100AH adequate for this purpose? (2). If 'yes', then how big is the size of MicroSD that I need?

Looking forward to hearing your responses.

The build in proxy does not cache dynamic content (like youtube etc) and therefore will not save you a lot of bandwidth. In addition, all of your users will be sharing the same source IP (of the proxy) which may or may not be an issue for you.