Hello. My name is, Mike. I'd like to try and assist you with your issue. Though I mainly provide PC support Word is primarily the same in both Mac and PC.
Typically in forms, fields are limited based on a maximum character limit and not the size of the box. However, you may want to try the following:
Right click on the text box and choose format text box. Then uncheck "Worwrap text to AutoShape" and uncheck "Resize AutoShape to fit text" and see if that limits your text.
Otherwise if know how many characters fit in the box then you can right click on the form field and choose properties and set the maximum character limit.
Hello. My name is***** can provide you with some additional assistance. I read through your question understand that you wish to set the width of a field or multiple fields based on space available on the page. While every field will have a certain length associated with it in terms of characters, you can use the space on the page to limit the size of the field. The best way to do this is to insert a table on the page and then use the table cells for fields. The cells will provide you the structure to the fields with relation to the form. Just use one cell per field and insert or remove cells from the table as necessary. Every cell has a width property that you can set. Please let me know if you still have questions.