What size are your sections?
So I started building my modular layout starting with the yard. I figured that’s a good starting point. Current size is 6x3 but was thinking of adding a 3rd to increase its size to 9x3. Is this excessive, not enough? Looking for some thoughts and opinions on good functional yard sizes in HO.
EJ&Keith;
From your post it sounds like each of your "sections" * is 3' by 3', is that correct? That's a pretty good size for your HO-scale layout.
You will be able to use these "building blocks" to construct any size, or shape, railroad you have space for. If you ever have to move, they will be easily transportable, so you will be able to save all the money, and effort, invested in your railroad, instead of scrapping it. Smart move! :appl: Who knows maybe someday you will be able to display your railroad at a train show.

You are quite right to start with the yard. It's likely to be the most complex bit of trackwork on your layout, and it makes sense to work on it first, take your time to do it carefully, and test it thoroughly, before moving on.
(*technically they're not "modules", though that term is very commonly used, and we certainly know what you mean. It's no big deal, just a minor technical point. Interchanging the two terms is actually so common that you are likely to see the word "module" used more often, for what is technically a "section'" than the actual, technically-correct term, "section" is.)
Strictly speaking, a "module" is built to match all the physical, and electrical, standards of some modular organization, like N-Track. Any module of a given standard length, and shape, can be substituted for any other module of that same length, & shape. For instance, there are likely hundreds of N-Track 4' straight modules all over the world. Any one of them could fit in place of any other one, be quickly connected, and have trains run over it. This capability of modules is important when Murphy's Law asserts itself at a public show. The various modular organizations usually have spare modules available to substitute for any module that, "experiences technical difficulties!"
In short, modules are portable, standardized, interchangeable parts, of one (often huge) temporary display layout.
A "section" is a whole lot simpler. It's just a piece of one model railroad period.
Sections are typically not interchangeable, and may well be different sizes, and/or shapes. Sections usually can only be assembled in only one pattern, to make up the whole railroad. My own railroad is sectional, and I heavily promote sectional construction for all model railroads.
Traction Fan :smilie_daumenpos: