Although you can’t format images in signature editor, you can copy and paste formatted images into the signature editor. Therefore, we can resize the image in message body, and then copy it into the signature. Please do as follows:

1. In the Mail view, create a new email with clicking Home > New Email.

2. In the new Message window, please click Insert > Signature, and then select the signature whose image you will resize from the drop down list. See screenshot:

3. Now the selected signature is inserted into the message body. Right click the image you will resize, and select Size and Positions from the right-clicking menu. See screenshot:

4. In the opening Layout dialog box, please type the percentage you will resize the images to into both Height and Width boxes in the Scale section, and click the OK button.

Note: You can also select the image you will resize, and then resize it with drag its edge as you need.

5. Now the image has been resized in the message body. Select the whole signature in the message body, and then copy with pressing the Ctrl + C keys simultaneously.

6. Click Insert > Signature > Signatures. See screenshot:

7. In the opening Signatures and Stationery dialog box, please: (1) In the Select signature to edit box, please select the signature whose image you will resize; (2) In the Edit signature box, Clear original signature content, and then paste the copied signature content with pressing the Ctrl + V keys simultaneously; (3) Click the OK button.

So far, the image in the specified signature has been resized already.

8. Close the message window as you need.

If you are familiar with HTML code, you can resize images in signature by changing the specified signature’s HTM file.

1. Open a folder in your computer, paste below folder path into the Address box, and press the Enter key.
Folder path of Outlook signatures: %AppData%\Microsoft\Signatures

2. In the opening folder, right click the HTML file of the specified signature whose image you will resize, and select Open with > Notepad from the right-clicking menu. See screenshot:

Now the HTML file of the specified signature opens in Notepad.

3. Press the Ctrl + F keys simultaneously to open the Find dialog box, type the image name (or any other keywords, such as <img )into the Find what box, and click the Find Next button until you find out the code about the image.

4. Now change the values of width and height in the code as you need.

5. Save the code, and close the Notepad.

So far the image in the specified signature has been resized.

Open the folder containing Outlook signatures

Import or insert HTML signatures in Outlook

Assign auto signature to every email in Outlook

Align or float images in Outlook signatures

Add or insert horizontal line in Outlook signature