When you want to use percentages:
Office.context.ui.displayDialogAsync(url,{ width: 30% , height: 60%})
When you want to use a specific number of pixels:
Office.context.ui.displayDialogAsync(url,{ width: 250 , height: 170})
Can you please reconsider ?
[Word,Excel]Support pixel of size when use dialog. · Issue #305 · OfficeDev/office-js · GitHub
This request was added to UserVoice but it wasn't migrated across to Tech Community.
officespdev.uservoice.com-34701499-allow-dialogs-to-be-sized-with-pixels-instead