What does a kernel of size n,n^2 ,... mean?
The point of investigating a parametrized problem is that we hope or assume that the parameter will be fairly small or even bounded by a known small constant in some particular case you encounter in practice. In that case, algorithms with FPT could be called efficient.
So yes, you can definitely pick an arbitrary parameter that will be very large in practice, but then a linear kernel will probably not be useful, because you are measuring the wrong parameter.
As a side remark, this is not something specific to parametrized complexity. In any type of complexity, you must be careful that the thing your are investigating the complexity of is actually meaningful.