The average human is 1.8 meters high and as such I set my units up in Max to 1 unit to 10cm. This allows me to accurately model most things. I also use some basic cubes to help me for door widths and heights and even window heights this is in various files that I load before starting a model or import in later to rescale it. You should be able to set your units up in whatever package you are using.
Just to add be careful you are not selling the same assets here that you are on Turbosquid, they can afford better lawyers than you and often use them. It is part of their artist agreement.
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