tp @e[x=-100,y=69,z=60,dx=230,dy=255,dz=200,type=minecraft:skeleton] ~ -200 ~

I wanted 100 blocks which would be 200 square spawn protection for my villagers and players so I made one for each type of hostile mob. My server spawn is x=0, z=160.
Apparently 230 blocks on the dx equates to 202 blocks linear on the x.

Mobs were still able to spawn (exist) within about 50 blocks of the center of the area. Specifically I only really tested that from x=0 to x=-50 around plus or minus 50 blocks of z=160. The code worked between somewhere around x=-50 all the way to x=-100. Testing in the positive x seemed to work fine.