If you have drop down archive in your blog and has an problem with its width then only read this or else you want drop down archive in your blog read this post.
Now i will tell you how to make drop down archive adjust to your sidebar.
Go to LAYOUT > Edit HTML > Expand Widgets > Press Ctrl + F > Now Search For Below codes
<b:includable id='menu' var='data'>
<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>
And Replace the above codes with below codes:-
<b:includable id='menu' var='data'>
<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>
The difference between the codes is only the red line we have added. I made you to replace codes instead of adding red line,so as you don't make any mistake while adding.
If you feel any problem while doing this leave your love in your comments, i will help you.
Best Of Luck !!