You can use a glow or aura effect to make your hero(es) pop and stand out.
Example of a hero glow from Warcraft 3:
How to do it in Starcraft 2
1. Place your hero on the map.
2. Enter the trigger editor. Make sure the "Show Libraries" and "Use Subviews" buttons are clicked.
3. Select the "Melee Initialization" trigger. Right click Action, select New -> New Action.
4. Select Label: Actor, Action: Attach Model to Unit.
5. Select Model: BlueHeroOmniLightGround, Attach Point: Origin. For the last "field", select Unit. Then click the Value bullet. Select your hero and click OK.
Voila! This trigger creates a "BlueHeroOmniLightGround" effect on your hero. Credit for this tutorial goes to s3rius, who attached a sample map below.
There are two sizes - Omni Light and Omni Light Large. If the large one is still too small you can go into the data editor, find the omni light model and increase it's size.
You might have to disable the size inheritance, or it could be that the omni light adjusts it's size to the model it's been attached to.
I found another solution: Using data only. One just needs a model, a model actor and a side operation actor to attach the glow (or some other visual effect) to a certain unit. The good part here is that actor events allow the modification of the models size and color and therefore one can use this for any number of different units.
@madmaxII: Could you elaborate on Data-only version? One of the main features of my mod is for anyone to be able to add it to any map as a dependency. I want to be able to do this, but have never been sure how.