I want to position a MessageBox in a particular position with respect to the active cell in a string grid and this is no problem using MessageDlgPos() except that I want to prevent the box running off the right or bottom of the screen when the active cell is close to the right or bottom. What I need for this is a way of getting the dimensions of the box but I cannot see a simple way of getting these. Anyone know how without creating my own box?
The MessageDlg...() functions do not support what you are asking for. The dimensions of the dialog are not known until the dialog is being displayed, and you have no way to access the dialog window directly to query/re-position it, except maybe with a WH_CBT hook from SetWindowsHookEx().
That being said...
On Windows Vista+ with Vcl.Dialogs.UseLatestCommonDialogs=true and Visual Styles enabled, the MessageDlg...() functions call the Win32 TaskDialogIndirect() API to display a message box. You have no control over that dialog's dimensions, so you would have to wait for that dialog to issue a TDN_DIALOG_CONSTRUCTED notification to then query its actual dimensions before it is displayed, so you can then adjust its position as needed. However, the MessageDlg...() functions do not provide access to any of TaskDialogIndirect()'s notifications (TCustomTaskDialog, which is used internally, does have an OnDialogConstructed event, amongst other events). So, if you wanted to reposition this dialog, you would have to call TaskDialogIndirect() yourself with a custom callback function (or, use the VCL's TTaskDialog wrapper).
On pre-Vista, or with UseLatestCommonDialogs=false or Visual Styles disabled, the MessageDlg...() functions display a custom VCL TForm via Vcl.Dialogs.CreateMessageDialog() instead, which you can call directly, and then pretty much query, manipulate, and show the returned TForm however you want. Just be sure to Free() it when you are done using it.