Try:
.x-body.x-gecko .x-btn-action-nav-large .x-btn-inner {
font-size: 5em;
}
You need a space between the last 2 rules as the inner element is a child of the button.
You could also simply do:
.x-gecko .x-btn-action-nav-large .x-btn-inner {
font-size: 5em;
}