The usable area of the zx spectrum screen was of 256×192 pixels (4:3) but there was a border around it. This border had little use, only its color could be changed. In it also appeared the red/cyan and blue/yelow lines when a program was loading.
Although there are “default” native resolutions, some games dont follow the rules, so its not 100% accurate unfortunately. Bear in mind some of those links in your first post show the res I wanted to display the output in, not the native resolution. For example I increased the GB to about 1.75 times the native size.