Landon, what is your intended use for the boiler? This will have some bearing on what size grate you are likely to need to produce the required amount of steam.
Generally a vertical boiler is easier to make than a horizontal one and firebox sizes are not that different for a given diameter.
Also approx what part of the world are you in and what book are you waiting for, there is a lot involved in designing a boiler so don't be in a rush to get going read the right books, talk to a boiler inspector if that applies to you or consider making to a published design.