Make all windows (Dialog Boxes) containing variable-sized content resizable
Too many windows force the user scroll well before the comfortable screen space for the window is exhausted. We also do not want these windows to simply be bigger and potentially take up an unnecessary amount of screen space.
For the best of both worlds, I suggest that a window should be resizable if and only if it contains variable-sized content (e.g. scroll bars, text boxes).
Examples of specific windows are listed in the comments.
Ingeborg Hawighorst commented
It is hard to work with formulas that reference ranges, like Sumproduct or array formulas, because the dialog shows only four rows. Make it resizable, so we can see more / all of the formula at once.
The current one is way too narrow. I know there is a work around for this where you increasing the size of one or more columns in the box then close it and reopen it but having it resizable like other dialogs would make so much easier.
Bill Bekenn commented
Yes, the Mac version automatically sizes most lists that are displayed to fit the width of the widest item, and as many items as will fit on the screen. This was always so even when early Excel was only available on Mac. When it ported to PC a lot of this was lost and things like the Drop Downs for Data Validation do not show more than a few items often needing scrolling, and they depend on the width of the column not the width of the items in the list. Mac version is still heaps better for dialogue lists.
Levi Bailey commented
And all other dialogue boxes and menus that contain contextual elements for that matter, including Evaluate Formula, Function Arguments, etc