This might help, not sure, hopefully I understood you, but I use this method ONLY when I am not using the city plans.
I use -REDACTED- as I found I had issues with increasing the limit via the Holotape on a previous version of sim settlement (Don't know about the lastest version)
I disable the option at the workbench that allows you to increase the build limit for any Sim Settlement when I use the city plan (If you are unsure, you can pick the option to notifiy you in 24hrs ingame just incase), but I tent to us 6x (Up to you how big you want to go though) for any custom built settlement, an example is Sunshine Tidings Co-op, I curently have that running with 24 residential plots and 24 various plots, i.e commercial, industrial, martial etc, including a fence all the way around, my own "over the top" gate protection system and seems fine.
The size budget is a polygon count designed to keep your frame rate steady by preventing you from building massive and overly complicated settlements, in my understanding, it doesn't just count for the buildings you have placed, but every single item you place, so you may not have many buildings, but you may have many items, easy to check, looks at the bar (I think Top Left) in workshop mode, if its full, you reached the limit.
