Thank you for your post.
At this time, FileMaker Go will not automatically zoom in a field. On an iPhone with a limited screen size, you need to double-tap the layout to zoom in, and then select the field to display the magnified area to position the cursor. For example, using the Sample Inventory.fp7 file, select the Record Detail-iPhone layout. Double-tap the "Item Details" tab to zoom in. Tap the Product Name field, and then you can use your finger to position the cursor.