/skin/common_files/modules/One_Page_Checkout/profile/address_fields.tpl
Around line 72...
Replace...
Code:
<input type="text" id="{$id_prefix}{$fname}" name="{$name_prefix}[{$fname}]" size="32" maxlength="255" value="{$address.$fname|default:$default_value|escape}" {$autofocus} />
With...
Code:
<input type="text" id="{$id_prefix}{$fname}" name="{$name_prefix}[{$fname}]" size="32" maxlength="{if $fname eq 'address'}32{else}255{/if}" value="{$address.$fname|default:$default_value|escape}" {$autofocus} />
That will set the maximum length of address line 1 to 32 characters.
Or you could just change
255 to
32 to limit
all fields maximum lengths to 32 characters.