Olga is right, and you did not provide a URL.
1) Appearance > Customize > Additional custom CSS code:
@media only screen and (max-width: 480px) {
.fb_reset {
display: none;
}
}
it will hide the button on mobile.
2) You can try:
.fb_dialog_content iframe {
width: 40px !important;
height: 40px !important;
}
#facebook .plugin svg {
max-width: 40px !important;
max-height: 40px !important;
}
if this doesn’t work, I can’t help with this, unfortunately.
3)
.fb_dialog_content iframe {
left: 0px !important;
}
4) I do not see such a possiblity.
Regards