It can be done using both script commands AND perm-monster spawn. Both require you sepcify the size in the Event label. ( 2 = Small, 4 = Big. Specifying 2 & 4 = 6, will result in 2 taking priority thus, they will be small).
Additionally, if you add 20000 to the Mob's ID, they will spawn as a 'BIG' type monster, meaning it's db size will now be large. So if you use only the event label, they will ' appear ' large, but will not count as large, unless you change the db either hardcoded, or via adding 20000 to it's ID.
eg:
mapname,0,0,0,0 monster Poring 21002,1,5000,5000,4;
The above will ensure that a Poring will spawn and appear to be large, and will be treated as if it's Database size is also Large, meaning Minorous Card (+15% dmg to large) will increase dmg to this proing, while Desert Wolf card (+15% dmg to small) will not.