Hi,
As I need lots of relays in my project and they have to switch a lot of times, I'm wondering if it is possible to replace relays with some smaller and more reliable parts?
In my project I'm using Arduino to control some 16 volts DC circuits, each circuit uses different amount of current, from milliamps to 50 amps.
Any suggestions?
MOSFETs perhaps ?
Or a combination of MOSFETs and DC SSRs with heat sinks for the heavier loads, perhaps? 
I often use some IRLZ44N
It's a MOSFET, that works with a 5v signal as the trigger. All you need, is a 1.2k resistor at the gate and voila !
Load can be up to 47 amps (with proper heat dissipation), voltage 55v. It's internal resistance is very low, so you barely loose anything across it.
For 50A you will need a beefier MOSFET than the IRL4Z44N. That's also gonna be a really bulky relay - probably more like a contactor, as switching 50A of DC is not trivial. A MOSFET Is usually a more suitable switch than a relay for such currents. Or maybe an IGBT?