Op says that 4.7" will be too much and first thing you guys do is recommend a 5" phone? I'm guessing that people aren't even reading the entire post... they see the title and jump right in, recommending whatever flavor of phone they happen to be using and adoring right now.
Black3ternity, it's true that Microsoft has been flooding the market with cheap devices. However, most of them seem to be using the same 5" panel. Alas, your choice is quite limited. 635 and 435 have a 4" display but both lack an LED flash. The 735 has an LED flash but a 4.7" display. If you want to stay below 4.5" then my suggestion would be the 635.
Maybe you should talk to you mom first and ask her what she really wants. In my experience delighting parents with new technology can go wrong quite massively
