Question Visa Vietnam

Hello

I am traveling to Vietnam in March/April. I apply for a Visum. Now I have a question.

I need the original passport, the approval letter, and so on. and 2 photos. I read that they need e size from 4*6cm.

Is this urgently necessary or better a bit smaller??

the on Arrival form Need it e smaller photo... Is the size very necessary? What is the best form?

Thank you for the answer.