Please add this code to Custom CSS in your store admin > Sale channels > Online Store > Themes > Customize > Theme settings so it could solved the issue on desktop
@media (min-width: 1025px) {
html .halo-popup.halo-size-chart-popup.customPopup-large {
max-width: 1400px;
}
}
Because the mobile screen size is small I recommend you use another image for your size chart; I recommend placing image above or below ‘Size Chart (in inches)’