In general, it is wise to make your plot files as large as possible so that the mining software does not have to wait for your hard drives to seek and find a bunch of small plot files each round. Having too many plot files on a hard drive could potentially slow down your scanning speed for mining. This rule applies mostly to HDDs because the size of plot files on SSDs has no significant impact on scanning speed due to SSDs having entirely different architectures and vastly increased speed overall as compared to HDDs.
When it comes to plotting large hard drives, I prefer to make all of my plots 2 TB in size. This keeps the total number of plot files low while also making it relatively easy to move the plots to any other 2, 4, 6, 8, or 10 TB drive as needed.