Resize and change text in ‘personalisation’ box

I am using twenty Fourteen with a child theme and WP ecommerce.

I would like to resize the personalisation box as it only needs to hold 5 numbers.

Also to change the text from ‘Personalize Your Product
Complete this form to include a personalized message with your purchase.’

To ‘please enter your membership number.’

Not sure which php files to change.

Thanks.

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