I think an 18″ wheel will fit into an ES250 standard swinging arm though I have never personally tried that. I have in the past successfully fitted an 18″ wheel to a TS250 which normally has a 16″ wheel. The only problem was the centre stand cross tube which had to be ‘adjusted’ to stop it fouling the tyre
A longer s/arm was fitted to the early ETZ250s (81-88) as they had 18″ wheels as standard but they are generally hard to find for some reason.
Bear in mind that if you fit an 18″ wheel the speedo will read incorrectly. You will also need to consider fitting a smaller gearbox sprocket as ES250s are already very highly geared. This also will affect speedo accuracy as on ES250s the speedo is driven from the gearbox output shaft and there are only two ratios available to the best of my knowledge (21/45 and 15/45).