Hi
From what you are describing you would seem to have an overlay playing. You will need to check your project. (Assuming you saved one.)
Go into time line mode,(as overlays are more easily seen). Now, when you click the green cross to add an overlay, (image/video/text,) whatever you enter (or select) appears on the overlay track as a short blue/grey box with an image icon in the centre, or, if you have added text, some of the text will be seen.
When you review your main clip, the red cursor line will, at some point will also pass over the overlay box. When this happens the contents will appear on your film in the preview screen. If you have not altered your overlay size, it will appear as a small "window" by default in the bottom centre of the screen. (An image will be static, a movie will play and text will display)
If this is the case and you want to remove it, then simply right click the overlay box and select to "Remove selected overlay/s) from the menu.
If you want to alter how it is seen then left click the overlay box to open up the control window for your overlay. In this you will find controls for its placement position (the arrow grid), scroll movement, size (as you have found), fade, opacity, border and chroma effects etc.
Note however, that you can add several overlays to the track and these can be overlapped so they play at the same time but appear as a single box on the track as by default they add to the cursor position. If this is the case you will only see the top one. You MAY have inadvertantly added your effect twice and whilst your controlsregarding size etc would work on the bottom overlay, the other one would remain as a default image and be seen in the preview.
Check if you have two overlays by grabbing the box and dragging it left or right. If there are two then they will separate.
This is just a thought but a possibility.
Nat