I just used the in-game settings to switch to windowed mode, and I double clicked on the window title area, and it maximized the window to my widescreen laptop display! The game now looks much smoother (not pixelated) and it's really widescreen!! Just awesome!! (I did not modify the game in any way, I use plain vanilla GOG RCT, as installed, that I recently bought).
And use the 'Borderless Gaming' freeware FORK by mdbell: https://github.com/mdbell/Borderless-Gaming/releases
to get borderless and fullscreen, though the game map does not scroll when the mouse is at the display borders - but use 'X-Mouse Button Control': https://www.highrez.co.uk/downloads/xmousebuttoncontrol.htm
- to easily map the mouse wheel and buttons to what you please to make up for it: I mapped my mouse whell to PageUP and PageDown, and set these keys in the game to zoom in and out, respectively, and now I have mouse controlled zooming! I mapped my mouse wheel (button) press to HOME and use that to rotate the game map, and I use my right mouse button to rotate the construction object, but for map scrolling I use traditional WASD..