Usually, and only a "rule of thumb" is that you can look at rectangular shapes in 4'-0" multiples and determine the square footage required (or even better is to map-out your machinery positions and desired work flow). Here's an example: a 40x40 building is 1,600 square feet and has 160 linear feet of exterior walls, yet a 20x80 building is also 1,600 square feet, yet it has 200 linear feet of exterior walls. That long shape yields more exterior walls - which in many types of construction equates to more building costs. However, good estimating requires a good plan and in some cases; a non-square rectangular shape is your best choice.
In a project such as this, you will need to determine where materials go-in and where your finished products go out... Example: some projects start with big raw materials and produce small finished assemblies - let's say you're using sheet goods (4x8) and producing dog houses for chihuahuas, then it is easier to move the small dog houses across the workspace than to trek through a maze with full sheets of plywood. The exact opposite can also be the case... Let's say you are building dining room tables, then you would bring-in dimensional lumber - yet outbound may be a large table! Many people build small shops very efficiently and have easy ingress and egress for raw goods and finished products. I know of a couple of small to medium sized shops that have garage doors at each opposite end. Also, make certain all equipment can easily fit.
Personally, at almost 60, now - I find tools and carts on wheels make a lot of the moving of machines, raw goods and finished products be a much easier task. I do have help (most much stronger than me), but we have found the wheel to be a great invention. Note: some floors are more conducive to rolling heavy carts and tools than others. Rough floors are best avoided, but still can be useable with pneumatic tires. Smooth floors work great with solid tires. Clean floors are also imperative with either type of tire. On big machines such as table saws, there needs to be a method to get the machine to a stable non-rolling state prior to using said machine.
I am no electrician - FAR FROM IT, but you should work hard to avoid the use of extension cords. I even have some electrical outlets overhead for a few of the machines that move-around. This is often the best way to keep cord tangles and trip hazards to a minimum. All of my guys are given bonuses to suggest and/or implement safety and convenience ideas - so as you can imagine, we run a very clean shop.
Dependant on how you work in your shop, you may need to acquire new shelving. Shelving can be store-bought or made to suit your needs. I have quite a few shelving units - because we have a simple understanding, "A place for everything and everything in its place". If someone misplaces a tool, it slows-down everyone and decreases out output, which in turn reduces income. What I have been using over the past 7 or 8 years is shelves that do not reach the floor! My favorite shelving units are made from vertical plywood hangers (attached to end of span floor joists) which, in turn support horizontal ledgers that support (select) Southern Yellow Pine stair treads. Everything is supported by square-drive screws. We like the nice rounded nosing that this material has and it is considerably stiffer than 1x12's. Having used SD screws means these units can be taken-down in a matter of minutes!
This manner of shelving is so cool for many reasons! It takes no floor space and can stop at any height - which means under it can be kept clean and available for small item storage - such as waste cans and many types of recycling. A couple of weeks ago, a client was visiting me and his grandkids wanted to see the reptile collection. These shelves were something he liked so much that he asked if I will do a basement full of hanging shelves at his new home. We have agreed to a deal and will be starting in Mid April. It is a huge house and the basement is also going to be quite a nice project. We bust our butts to keep our customers happy with us and projects like this do pop-up every so often. It will be a nice change of pace!
I hope this helps!
Otis Guillebeau from Auburn, Georgia