If the design is limited to being similar to Real World scarecrows, it will be roughly human sized, appear to be made of sticks and straw stuffing clothes.
If based on real world scarecrows, Scarecrow units may have little or no move or attack, making them exclusively garrison units serving as sentries. To compensate for this, they may have natural Lookamancy to pierce Foolamancy or natural Shockamancy to harm or demoralize enemy troops. If based more on the Scarecrow from the Wizard of Oz, Erfworld Scarecrows may accompany armies in addition to garrison duties and serve as combat support troops; not engaging in direct combat, but using Lookamancy and Shockamancy to support the troops who are doing the actual fighting.
Real World References
In the Wizard of Oz, one of the three main supporting characters to Dorothy was the Scarecrow. The Scarecrow had no brain, and sought the Wizard to try to get one. Dorothy, like Parson, was pulled from her World (which was suppsed to be ours) and into an alternate reality, Oz, where magic existed. Oz bears many similarities to Erfworld, but the greatest difference is that it is not a game world. Parson's statement before leaping through the portal near the conclusion of the Battle for Gobwin Knob, "There's no place like home," was the phrase Dorothy used to return to her own world. Parson was, obviously, not so lucky.