Hi SophieAdams30, and welcome to Scarleteen!
It's actually a myth that blood is an expected part of having sex for the first time - usually, the presence of blood means that the tissues of the vagina have been torn or injured, because of a lack of lubrication or too much force. So it can happen anytime, not just the first time that you have sex. The idea that you have to bleed the first time you have sex, or even the idea of virginity, were socially constructed by folks who didn't have a whole lot of knowledge or care for vaginas.
Magical Cups and Bloody Brides: Virginity in Context gives some really good background info as to how those myths came around, and how they can be damaging to our modern day expectations surrounding sex and our bodies. The hymen, for that matter, has a whole lot of myths and misconceptions surrounding it, so you might also enjoy reading
My Corona: The Anatomy Formerly Known as the Hymen and the Myths That Surround it.
As for your partner's size, it's possible that it could factor in, but the vagina tends to expand during arousal, so barring extreme exceptions, it doesn't tend to cause issues unless there's something else going on (i.e. not enough lubrication, not aroused enough, taking things too quickly).
With Pleasure: A View of Whole Sexual Anatomy for Every Body goes into more detail about how exactly our parts change and play a part in arousal and sexual pleasure.
And now, a couple of questions for you - how did it feel when you had sex with your first boyfriend? Good? Painful at all? How about your boyfriend now?
Also, are you using any kinds of contraceptives or barrier methods if pregnancies or STI transmission are a possibility?