Although you can't resize an image arbitrarily, there are different sizing options.

Image block

The most common type of images are "image block". They're full width images with a caption. You can decide to align them to the left, center. You can insert an image block from the command palette:

Inline image

By default, inline images are sized proportionally to the font size. It's especially great for inline badges and icons, among other things. When inserting an inline image, you can also set it to its "original size". By doing so, the image will remain inline but with its original size until a maximum width. To create an inline image from the "plus" menu appearing when hovering your caret:

You can change the inline image's size by selecting it, a menu will appear above it. Note that you can also turn an inline image into a block image.