Took me a while to respond as I had to make an account and wait for my email to be confirmed! ; )
I had the same problem as you mate, speedial thumbnails way too small on large monitor.
It has already been said ............. increase tile size.
Here's how I did mine >
Top right of Opera browser, there's a few little lines that look like a graphic equalizer
click that > Easy Setup, appears
Under 'WallPapers' there's a heading for 'Appearance'
Activate 'Use Bigger Tiles'
Job done! ; )