How can I truncate the HtmlBody when sending emails through postmark

Question

I'm using postmark to send emails in my rails application where the email content is auto generated. The thing is that in some cases I getting this error from postmark.

Postmark::InvalidEmailAddressError

Invalid 'HtmlBody' value. It should be less than 5242880 characters in length.

How can I truncate the HtmlBody content in order to avoid this error using ActionMailer