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By default, the size of tiles is  proportional to  to the video resolution.
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To fix the size and expand tiles to all free space on the layout area, do as follows:

1. Go to  Settings → Options → User  Options (1) → User interface (1–2). 
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2. Uncheck the Adjust video frame border checkbox  checkbox (34).
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3. Click Apply to save the changes.
4. Reopen the Layout (see Layouts Management).

The video will retain its aspect ratio.
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To expand video to all window, do as follows:

1. Uncheck the Keep aspect ratio checkbox (3).
2. Click Apply to save the changes.
3. Reopen the Layout (see Layouts Management).

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