before make sure first widged archive you are already using a form dropdown, because if not then the code which we will search not found, ok just here is how:
1. Enter your blog account.
2. then choose template.
3. click on edit HTML.
4. find the code as below, as always use CTRL + F to search for it.
<b:includable id='menu' var='data'>then add the code style = ' width: 100% ' after the code " _ArchiveMenu " ' and will look like the example below
<select expr:id='data:widget.instanceId + "_ArchiveMenu"'>
<option value=''><data:title/></option>
<b:loop values='data:data' var='i'>
<option expr:value='data:i.url'><data:i.name/> (<data:i.post-count/>)</option>
</b:loop>
</select>
</b:includable>
<b:includable id='menu' var='data'>The last step IE click Save and view the results with a new tab, in the event of error check back whether its placement is correct, use the CTRL + Z to review it, the tutorial today and good luck friend bloggers.
<select expr:id='data:widget.instanceId + "_ArchiveMenu"' style = ' width: 100% '>
<option value=''><data:title/></option>
<b:loop values='data:data' var='i'>
<option expr:value='data:i.url'><data:i.name/> (<data:i.post-count/>)</option>
</b:loop>
</select>
</b:includable>