Thank you both for your replies!
The fourth byte is 31 for all 20 sprites, so it couldn't be there, as they would all be the same (either small or big).
But your help was the catalyst for me finding it!
First of all, I was expecting value AA anyway, as AA is 10101010 and there were 5 (more than 4 I mean) big sprites. So, I was looking for AA in the rom, near the area of the bytes of the sprites. There was one AA byte (at 69CB) which was affecting all sprites, but I couldn't understand how and find the solution. Changing any other bytes near it was just corrupting everything (plus, changing this byte was affecting ALL the different words). When Reld said "lda #$aa", which is "A9 AA", I realized it must be the suspicious AA byte, as it was preceded by A9. Then I realized that there must be another A9 xx somewhere after it (instead of a xx byte without AA before it, as I thought at first). Well, 4 bytes after the AA there's an A9 02 and after another 4 bytes there's an A9 00. In fact, because there's a big distance between them, I hadn't found them by looking at them.
So, without the bytes between, we have A9AA A902 A900 A900 or AA020000, if we omit the A9s.
If we see that in binary:
4 3 2 1 8 7 6 5 12 11 10 9 16 15 14 13
10 10 10 10 00 00 00 10 00 00 00 00 00 00 00 00 ...and so on
In the first row I have the sprite order*. So I need to change them to AAAA0A00 (10101010101010100000101000000000)
Of course it affects all words (Pause, Bonus, Play Off, etc.), but as it makes the sprites bigger, I can fix all of them one by one (I'll just rearrange the sprites bytes to show blank tiles).
*The bytes order above is not exactly like this. It was very confusing when I had to change the sprite bytes. I think the right order for AA AA 0A is really AA 0A AA for some reason, making the sprites 4 3 2 1 12 11 10 9 8 7 6 5 (probably, I didn't need to experiment more).
Now, I just need to try to center the sprite and then fix all the others, that now show extra tiles.
Again, thank you both very much for your replies, you saved me a lot of time and frustration!
Last night I searched the whole forum for anything similar. There were two or three discussions about how to make sprites bigger, but not this way and I think only for the NES. Changing the small sprites to big ones, means you can make the game sprites bigger! In fact, you can add 3 more tiles per small sprite (or more, if the big sprites are bigger than 16x16)!
As in the past I had done translations only, I never needed such a thing, but now I think it should be a common method to make sprites bigger. There are so many graphics hacks and I don't know if any of them do that.