I saw somewhere logo files just like the png file in /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.pn but much bigger. Where can I download them ?
It would have been nice, if you had actually shown the image you are looking for, because I do not have that file on my computer!
But assuming that the title of your question is correct, then the Ubuntu logos are here. If you need a large version, just download the svg and create your own png with the size you need.
There is also an explanation about how to use the logos.