433mhz antenna

What is the best antenna length for 433 MHz?

For 433MHz frequency using a ¼ wave whip the ideal length is ~173mm as a straight length . Not more and not less. Shorter or longer will still work but if it is not an exact ¼ wavlength the antenna impedance changes and you do not transmit the maximum power.

How can I boost my 433 MHz signal?

If it's still not working try some better antennas, those short stubby 433mhz antennas aren't very good. You can build good 433mhz dipole antennas really easy. Get a SMA to SMA cable (assuming that's the connector on your tx/rx) and cut the coax in the middle (so you can make 2 antennas) then you can solder some up.