A little song from the musical "Little Miss Broadway" (1938). The title comes from the traditional nursery rhyme of the same name (see here).

I can't show how much I love you
In this little space I've got
I don't know how much I love you
But it's an enormous lot

If all the world were paper
And all the seas were ink
I'd write a great big note to you
And tell you what I think

I'd say "I love you dearly"
In letters three miles high
And sign it "Yours sincerely.
Cross my heart and hope to die!"

If all the stars were diamonds
In golden skies above
They wouldn't be worth that
Without your love

I'd like to write "I love you"
So big that you would blink
If all the world were paper
Yes, and all the seas were ink

AN ENORMOUS LOT= Very very much, a real lot. (enormous = huge, very very big).

IF ALL THE WORLD WERE...= In conditional sentences, in the if-clause, the verb TO BE uses the old subjunctive form (I were, you were, he were, we were, you were, they were). In modern English, nevertheless, it is also common to use the normal past tense, so you can also say "If all the world was paper".

YOURS SINCERELY= An expression we use at the end of a formal letter, before signing our name.

CROSS MY HEART AND HOPE TO DIE= This is what children say when they want to make a serious promise. They say this while drawing an imaginary cross with their pointer finger on their heart.

BE WORTH THAT= Have that value (in this case, she said that if the stars were diamonds, they wouldn't be worth that, so she means that the stars would not be as valuable as diamonds are... without your love)