If you place a base cabinet, for example, the default width will be 30", so you can select it and change the width as desired (28", as you stated).
However, there are 2 situations where the rules are not the same.
1 - Cabinets with appliances: if you place a fridge cabinet (width= 36") you can't resize the width to 30", because this is less than the width of the fridge.
2 - If the wall space is not enough for the width of the cabinet. So, if you have a wall with length= 28", you can't place a cabinet with 30", independently of the fact that you are going to change it. There are some tricks to solve it.
A few options:
1 - Don't insert the cabinet close to the end of the wall => place it one or two feet towards the interior => select the cabinet => resize as desired => drag the cabinet along the wall => you will be able to place it at the end of the wall
2 - If you have a particular situation where you can't drag the cabinet along the wall, you can use a cabinet object. There are several cabinets and bookshelves in the furnishing library.