How to Resize a Notion Embed
Hover over the embed and handles appear. Drag the bottom edge for height and the side handles for width. That is the whole mechanism, but two things about it catch people out often enough to be worth stating: sizes are stored per page rather than per widget, and width is capped by whatever column the embed is sitting in.
Sizes are per page
The same widget can be tall on one page and short on another, and resizing it in one place does nothing to the other. This is almost always what people mean when they say the resize did not save.
Width is capped by the column
If the block refuses to go wider, the block is not the problem, the column is. Move the embed out of the column, widen the column, or turn on full width for the page from the menu in the top right.
No numeric input
Notion has no size box, only dragging, which is fine on a laptop and genuinely fiddly on a phone. Set sizes on a big screen and check them on a small one.
If it will not resize because there is nothing in it, that is a different problem and it lives in widget not showing. Phone behaviour is in on iPhone, Android and Desktop.
Resizing in other tools
Notion is unusually generous here. Coda and Craft let you drag as well, but Google Sites and some WordPress themes fix the frame height, so a widget that needed dragging in Notion may need a height set on the widget itself instead.
If the same widget lives in more than one place, build it to look right at a middling height rather than a tall one. It degrades better, and it is the safest habit for anything going into a template, since you cannot resize somebody else's copy.