Answer: Leyland Cypress will fill in sideways naturally without trimming. Its normal shape is somewhat pyramidal so it is naturally wider at the base. And, it stays dense from top to bottom when grown in full sun. So there is no need to trim the sides.
If you need to control the height, it is best to begin trimming the top before it reaches the maximum height desired. Keep in mind that this is a strong grower to about 25 feet or so, and you will need to prune it often to keep it within a given height range. For ease/practicality of maintenance, you may want to consider how high you can prune safely from a ladder or with whatever tools you plan on using.
As far as fertilizer, you would not need to fertilize if it seems to be growing well. (Often, a planting adjacent to a lawn receives ample fertilizer when the lawn is fed, for instance.) And, since this is a robust grower, you do not want to encourage excessive growth. If you feel it is growing slowly, you could work with your local county extension to run some soil tests and work up a fertilizer plan based on the results.
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