Si4012 loop antenna question
Not being much of an antenna designer the way I normally go about it is to copy a reference design which most RF chip-set manufacturers have available. If you go to the Si4012 Page page and look under the Tools tab you'll see reference designs for 434, 868 and 915 MHz antennas at the bottom of the page. Those files contain PDF and Gerber files containing a variety of designs:



You'll also want to read the AN727: Si4012 Antenna Interface and Matching Network Guide that includes worked out matching values on page six for common frequencies. If you need to design your own antenna or want some more detailed information about it they have the following application note that describes the design process in greater detail:

AN639: Design of Printed Trace Differential Loop Antennas

I noticed on the tools page they have the complete designs for 434 and 915 MHz TX PICO boards that would be worth a look. It also looks as though they're available as development boards at reasonable prices which might be worth considering so you can evaluate them initially and have a known good design for comparison.