Is there a specific size of phone that fits in the controller? Will an iphone 5 fit?

Yes it'll fit, but the 5 might be a little underpowered

iPhone 5 is not a supported phone, though the iPhone 5SE is. Screen is awfully small, though -- after long resisting a move to a larger phone, I upgrade my 5 to a 7 when I ordered my Mavic a year ago and have been quite happy with the change.

The Mavic Air remote controller can fit smartphones between 6.5-8.5mm thick and up to 160mm long, without a phone case. iPhone 5 will fit but it is not in our compatible list, the compatibility cannot be fully guaranteed, appreciate your understanding.

You may want to buy a 6 or 6s off of eBay now that the 8 and X are out.  Their prices are much lower than they used to be.  I use a 6s Plus and love it.

The Ipad Mini 4 is the way to go if you have the funds

Bigplumbs Posted at 2018-2-5 00:15
The Ipad Mini 4 is the way to go if you have the funds

The Mini doesn't fit in the remote's phone holder very well, does it?  Seems like a very tight fit, IF you force it in.

I had an SE and loved the size of a small phone until I got the mavic then I wanted something with a bigger screen, now I have an iPhone 8 and its perfect for me.