Yep, I’ve had this issue. I didn’t have examples of heading sizes before the upgrade so basically looked at the post title size I have set (which is an h2, and set to 24px in my case) and then looked at the headings I see in Visual mode in the TinyMCE editor relative to that 24px and took those, and added this to custom CSS. It’s a hack and I don’t like it but it looks better. Your preferred font sizes will differ to mine of course.
This is my workaround and I suspect Cryout’s will be better.
.entry-content h1 {
font-size: 32px;
}
.entry-content h2 {
font-size: 24px;
}
.entry-content h3 {
font-size: 18px;
}
.entry-content h4 {
font-size: 16px;
}
.entry-content h5 {
font-size: 13px;
}
.entry-content h6 {
font-size: 10px;
}