Thank you for yoru post.
This issue was reported by another member on the FileMaker Go board:
I have attached your post to the original issue.
From the related post, the current workaround is to add a vertical scroll bar to the field. That way, it is restricted and displays one line height, and FileMaker Go does not display the vertical scroll bar.
In layout mode select the field and in the Field/Control, Display as Edit Box, then click to Include vertical scroll bar. Field will not enlarge upon entry.