>HI,
>
>I am testing an email that has HTML built on Bootstrap sent and received to/by my iPhone.
>
>What I notice is that initially, the letters/fonts appear small. Then I turn the iPhone 90 degrees (place it it horizontally); then turn back to vertical position. Now (after the turn) the font appear larger (desired effect).
>
>Could anyone see/suggest why? Why not the fonts are "large" to begin with?

Try to add this line into the head of your page:

<meta name="viewport" content="width=device-width, initial-scale=1">

That should tell the browser that you're handling the sizes in css and wherever, and it shouldn't try to outsmart you.