The default size of a select box is determined by the max length of the option labels. If for instance your options are
10
20
30
given that the field adds 'Choose...' as the neutral option, the default size would be the length of 'Choose...'.
You can use css to set a smaller width to the element, in which case the 'Choose...' will be clipped when the box is collapsed.
If you want to replace the 'Choose...' with something else, e.g. '...', you will need to use javascript.
hth