You can globally modify unit size on strategic/cloth map from game options in Advanced Options, near the bottom.
The scale of a race is trickier to add, because there's a bunch of problems with it. There are two values that you should care about.
The first is <ModelScale>1</ModelScale> found in each UnitType. The problem with changing this value to anything but 1 for designable units is that mounts do not get scaled by this number on strategic map, but they do get scaled in tactical map by this value. So unless you are only slightly modifying it, you will notice strange mount differences.
The other option is UnitStat_TacticalScaleMultiplier, a UnitStat that has DefaultValue 1.0 ( = 100% of <ModelScale>). This value only dictates the unit's size in tactical combat, not in strategic mode. So you can make a unit appear much larger in tactical battles without making them look silly in strategic mode. The problem with this value is that the game expects it to be 1.0 for the spells that affect that unitstat (Shrink, Shrink_Ability, Growth, Growth_Self, Giantform). So unless you are only slightly changing it, you may want to consider how those spells are going to impact the scale of your units. For example, in Stormworld, my Frost Giants use UnitStat_TacticalScaleMultiplier to look big in battle, but I also modified those spells to not mess up their size.