What would you call size that fits between small and medium?

Our UI supports small (32px wide), medium (64px) and large buttons (96px).

Now need appeared to add 48px wide buttons and it would be nice not to rename all the rest, just to give them a name which will fit them between 'small' and 'medium'. Can you suggest that?

Informal names are okay.

As a neologism, it can be smedium

There are urbandictionary entries also. Here is one:

when a size small is too small and the size medium is too big you are a size smedium!!

They don't have my size I can't fit the small or the medium if only they had a smedium!

Other than that, you can use different degrees of small like extra small. So small fits between extra small and medium.

Note: I treated the question as context-free as well for future reference. You can also get useful answers about user interface/technical approaches on ux.stackexchange.com

Keeping the same convention, combine the two?

Small, Medium-small, Medium, Large

I go with "medium-small" (with or without the hyphen).

The size between medium and small is like the direction between North and West (Northwest)

Similarly, the size between Medium and Large is Medium-Large,

This also fits nicely with how we order steaks: Rare Medium-Rare Medium Medium-Well Well Done

Several user interfaces use compact as another word for “small.” For example, Gmail and Google Docs offer compact controls as a slightly-tighter-than-medium setting. It’s also an established term for small cars.

smaller might be OK as long as you take medium to mean 'normal'.

Not quite the same as CSS font-size 'smaller' but I think the meaning is clear, it's not medium and it's not small.

smallish if you prefer.

Learn the lesson not to call 'styles' by a common enumeration, small=size32, medium=size64, large=size96. Then you can add in as many different sizes as you want, you can easily slip in a size48 without worrying about any knock on effect to other styles.