change /usr/share/plymouth/themes/elementary/logo.png and logo_blurred.png
a proper size is like this:
root@Vaio-VGN-UX490N:/usr/share/plymouth/themes/elementary# identify logo.png.orig
logo.png.orig PNG 536×536 536×536+0+0 8-bit sRGB 47.4KB 0.000u 0:00.000
then run: update-initramfs -u