If you have previous day's data, I would not expect it to change much if you have the time period set to a fixed value (2 hours, 6 hours, etc). If you have it set to current day only, it may look a bit large at first.
If you want them a constant size vertically, you need to right click on the scale and instead of selecting Dynamic, select "Sliding Window" then specify the Y axis range that you want the sliding window to have. Otherwise, QT will dynamically size the candles to best fit the window based on whatever data you have.