Typora allows you to use <img> tag for displaying images, which can also be used to adjust the size of images.
For example, you could specify the width or height attribute of an <img> tag, or set the width/height in its style attribute:
<img src="https://www.google.com/doodles/kamma-rahbeks-241st-birthday" width="200px" />
<!--or-->
<img src="https://www.google.com/doodles/kamma-rahbeks-241st-birthday" style="height:200px" />
Another common use case is that when you insert a retina image, you need to scale it to a “correct” size. To do this, specify a zoom factor in its style attribute:
<img src="https://www.google.com/doodles/kamma-rahbeks-241st-birthday" style="zoom:50%" />
You can set other css properties in the style attribute: they will be ignored when you edit or preview by Typora, but can affect the exported HTML or PDF.