Use a text input font size of at least 16px. If your text inputs have a smaller font size than that, iOS browsers will zoom in on the left side of the text input, often obscuring the right side and forcing the user to manually zoom out after using the text box.

When picking a base size for a desktop website or web app, you can break down most designs into one of two types:

Text-heavy pages. Articles, blogs, news, etc. These are pages where the primary purpose the user has on the page is to read. There is very little in terms of interaction – perhaps just clicking a few links.
Interaction-heavy pages. Apps that involve more hovering, clicking, searching for an item in a list or table, editing, typing, etc. There may be plenty of text on the page, but it’s not stuff you read straight through like a book.