アイフォンの画面が小さくてタイプするのが大変って英語でなんて言うの？

If you want to explain that typing on a phone is difficult because of the size, you can say:
- The screen on my iPhone is small, so it's not easy to type.
- My phone's screen is too small, so it's kinda hard to type.

As for which is better, "on/with/in," you can use the first two:
- It's not easy to type on my phone.
- It's not easy to type with my phone.

"It is difficult for me to type because I am on my phone" and "I am using my phone at the moment so it is difficult to type" both these explain that you are having a difficult time to type because you are on your phone.

It is difficult to type on my phone because the screen is small. explains the reason it is difficult to type is because the screen on the phone is small.

Just tell them that "It's not easy to type on a small screen" or simply that it's "complicated". That alone explains it all.

"Typing on a small iPhone screen ain't no walk in the park" this expression is for you if you have a sense of humor and you want to sound a little sassy and funny because when something is "no walk in the park" it means that it's not easy!

By saying it is hard to message the person will know you mean type.

Both phrases can be used in this situation.

I can't type on my iPhone because the screen is to small. - This is clearly indicating that you cant use your iPhone to type because of the small screen.

I am using My iPhone and the screen is very tiny to type on. - Tiny is very small. This is clear that you are using your iPhone and that you can't type on the tiny screen.

1. You may be in your office with a pc in front of you, speaking to someone on your iphone and also typing characters onto your PC, "Sorry, I'm taking so long. It's difficult typing when I'm on the iphone." "That's OK, take your time."
2. You are in the street speaking on your iphone. You want to give your friend someone's email address - which is in your iphone's memory - and you are typing the address:
   "Texting is not so easy whilst speaking on the phone."
   "Yeh, I know. There's no rush - I can wait."

To tell someone that it is hard to type because you are on your iPhone, you can say:
"I am struggling to type because my iPhone screen is too small."
"I find it difficult to type on my iPhone."