Vague questions often deserve vague answers.
As you may know, W and L are respectively the idealized dimensions of the width and length of the MOSFET channel. A basic reasoning (without further researches) would lead to the conclusion that increasing W would decrease its "ohmic" effect and increase its current capacity, while increasing L would have an opposite effect, but on the other hand would increase its breakdown voltage. A more elaborated study requires modeling with an approach to the physics of electromagnetism fields applied on these materials, and from there arise the descriptive equations that you find everywhere, in which case you can extract the other effects, such as propagation time, dissipation, capacitance, etc. Like everything in the world, there are compromise relationships, where increasing one factor impacts in another aspect, and in the end the goal is to harmonize those parameters which is achieved by providing an optimum W/L ratio. I believe you did not open this thread just to ask this, since this basic information would be a requirement for introduction into that discipline at the university, therefore be specific.