I'm not sure if this is even an issue with your addon as scaling works properly with icons on the actionbar but with TellMeWhen (TMW), the font size on the addon's timer text is very small and hard to read. It's not possible to override it in either addon and while I can disable OmniCC and have the default wow timers scale fine in TMW, having OmniCC enabled over rules any caling in either addon and does what the attach screenshot shows:

I am using TMW version 7.1.2-18-gf2099f3
OmniCC 6.0.4

Disabling automatic scaling in OmniCC does not resolve the issue.

again, scaling works fine in the default actionbars or in my case, bartender and in buff aura icons as well as Pitbull unit frames. It only does not scale to fit or register the font size in TMW.

The proc/count text in TMW is showing the correct size as I believe it does not hook into OmniCC, only the timer text is having this issue.

It's working fine for me:

Is now lol

I still have this issue, specially when the frame is scaled less then 1.
The addon here is CT_BarMod.
It's working fine with 6.0.1 version, but broken any version after it.

The numbers are as small as for the rechargeable spells.

Adding that for now I can create a group and set "TellMeWhen" as the rule and just set the base font size very high to get it to scale better.

Incase anyone else comes across this issue and until it can be found if this is a bug or not.