You can increase the font size in ProtonMail by adding this custom CSS. Just remeber to edit the number values with your desired sizes.

h1 {
font-size:24px;
}

h2 {
font-size:22px;
}

h3 {
font-size:20px;
}

h4 {
font-size:16px;
}

h5 {
font-size:14px;
}

h5,h6 {
font-weight:700;
}

h6 {
font-size:12px;
}