Question

I am planning to build this model, but how do I know how big to make L1?

What circuit analysis techniques should I perform against the PNP transistor?

Which components directly affect the output frequency?

http://circuit-zone.com/ediy_blog/724/simple-mp3-fm-transmitter.jpg

Answer 1

Replace the circuit with it's signal model, ie remove the DC bias.
The input 10K||10nf works as a input low-pass filter
5pf is use to short out gain at high frequency
L1 in parallel with variable capacitor determine frequency, the Q of L1 will determine bandwidth.

Answer 2

Actually its and NPN BJT transistor, the arrow points to the n-type material, and there's several models you can use for it.

Answer 3

The stereo input plug probably inputs audio, generally from 20-5K Hz and is used to amplitude modulated the out frequency to transmit onto the antenna. You'll have to do a full analysis of circuit to determine audio bandwidth and carrier frequency. According to FFC regulation your transmitter must be under 1 watt if not licensed.