I've recently built a relatively big Tesla coil. It is a larger version of the Slayer exciter but with 2 big MOSFETs. The same circuit, with a smaller coil produced small arcs without a top load, and bigger arcs with a top load. My problem is that, with a larger secondary coil, the circuit produces large arcs without a top load, but when I add a top load, it gets overloaded and the arcs get very small. Can I solve this problem by making the secondary coil shorter? It is quite long.
This is an answer in the making, please don't accept it yet.
This is not a discussion forum, and there is the risk of this question being closed as unclear, but let us cut you some slack and attempt to help you ask a specific question what is destroying your MOSFETs.
There are many many ways to make a Tesla coil, transformer based as yours, primaryless half-bridge, resonant tank, distributed resonant tank and so on. You need to provide a schematic of yours.
Please install LTspice and create a schematic of yours. The coupling factor between the two coils, needs to be estimated or measured. You'll need to add your input voltage, exact make and model of MOSFET and so on.