Google is not able to import individual email messages that have attachments larger than 25 MB. To help your migration go smoothly, look through your email and detach (i.e. delete) attachments larger than 25 MB.
In Thunderbird, click on the right-most icon in your message display. It should look like a little box, or a small grid:
This will give you a drop-down list. Click the “Size” display option:
This adds a clickable header to your message display, which allows you to sort emails by size. Look for messages that are larger than “25 MB” or “25000 KB”.