how can i query the size of an opencl kernel argument?

There’s no standard way to check before setting the argument as far as I’m aware, but the clSetKernelArg call will return CL_​INVALID_​ARG_​SIZE if the sizes don’t match properly, so that should allow you to detect and handle errors accordingly:

CL_INVALID_ARG_SIZE if arg_size does not match the size of the data type for an argument that is not a memory object or if the argument is a memory object and arg_size != sizeof(cl_mem) or if arg_size is zero and the argument is declared with the __local qualifier or if the argument is a sampler and arg_size != sizeof(cl_sampler).