Unfortunately, I am not aware of any options that would allow me to control the vertical size of the enclosing region in CreateDialog and DialogNotebook. It seems that the size is not related to the value that is set in WindowSize for either of the options.
For example,
CreateDialog[
{
Grid[{{""}}, Frame -> True, ItemSize -> {Scaled[1], Scaled[1]}]
},
WindowSize -> {200, 100}
];
creates a dialog of the correct size. However, the vertical size of the enclosing region does not seem to be consistent with the size of the window:
The behaviour persists if DialogNotebook and CreateWindow are used instead of CreateDialog:
CreateWindow[
DialogNotebook[{
Grid[
{{}},
ItemSize -> {Scaled[1], Scaled[1]},
Frame -> True
]
},
WindowSize -> {200, 100}
],
WindowSize -> {200, 100},
WindowMargins -> {{0, 0}, {0, 0}},
ScrollingOptions -> {"VerticalScrollRange" -> Automatic}
];
Note: I am using Mac OS X Sierra 10.12.5 and Mathematica 11.1.1.0.
EDIT
Following a comment that was made by @Kuba, I thought that a potential worakaround could be to provide an embedding element with the fixed image size to Grid:
CreateDialog[
Pane[
Grid[
{{"qqq"}},
ItemSize -> {Scaled[1], Scaled[0.8]},
Frame -> True,
Alignment -> Top
],
ImageMargins -> 0,
ImageSize -> {200, 100}
],
WindowSize -> {220, 120}
]
However, unfortunately, the result was identical. The result was also similar when I tried to use Framed instead of Pane.

Scaledis always a pain because they do not respect different types of margins/frames in ways one would expect. So it works correctly, it just ignores cell margins. I'd go with something like:CreateDialog[Framed[ {""} , ImageSize -> {200, 100} ] , WindowSize -> All]. Keep in mind that The very bottom of CellMargins is missing. – Kuba Jun 01 '17 at 13:36ItemSize. However, my question does come down to the ability to control the size of the enclosing object of theGridthat is embedded inCreateDialog. The horizontal size of the enclosing object seems to be governed through theWindowSizeofCreateDialog. However, I could not find a way to control the vertical size of the enclosing object. Also, sorry for deleting the question with your comment - I decided that it was, effectively, duplicate/specialisation of the question in the link that you have posted. – Jun 02 '17 at 12:34Framed, it is because Cell's height is automatically adjusted to fit contents, so the full height will be of height of the notebook, but then cell margins are ignored and you have misaligned content. – Kuba Jun 06 '17 at 13:20Griddoes not seem to recogniseFramedorPaneas its enclosing object, even if theGridis located insideFramedorPane. Please see the EDIT part in the formulation of the question. I usedPaneas an example, but the effect is similar whenFramedis used (the result is not entirely identical forFramed, but the vertical size of theGriddoes not seem to be identical to the vertical size of the enclosingFramedwhenScaledis set to1. – Jun 06 '17 at 14:10