When I bought a new computer, the display size became bigger and the icons became relatively small. Is there a way to adjust the size?
Not manually, but most scales can be set with a theme.
Try switching to the "large" theme via the display settings. If that's not big enough, you can try the large version of the p192c-theme (found here: https://github.com/Flemmbrav/Pak192.Comic/releases) - it's much bigger, but looks quite different from the standard theme.
Such a recent version of Simutrans should also be able to scale up everything without a theme, but only 200% or 300% and so on. I think it requires a command line argument (at least it used to), so not the easiest for those who are not used to using the command line.
I was able to change the size by selecting the theme from the display settings. Thank you very much.