In Outlook 2013 (& 2010) if you paste or insert an picture (e.g., Logo) in Signature Editor, it is not resizable. You are stuck and not able to do a correct sized picture in Logo. But No worries. There is an very simple trick that you can make the signature however you want.
- Click New E-mail in Outlook 2013 (or 2010)
- Type and insert the picture and make the signature that way you want.
- Now Copy the whole thing in the compose window.
- Click Insert –> Signature –> Signatures..
- Click New and type a name, click OK.
- Paste the copied signature
That’s it. Enjoy. 
Anand, the Architect 