If you have answered the first three questions, this one should answer itself. If you have decided that you want to create a niche firm specialising in a particular area of law, acting for a relatively small market segment, and those clients will instruct you because of that expertise, you probably do not need to create a huge firm. However, if you want to create a firm acting for a wide range of commercial clients, across a wide range of market sectors, then you need the scale your business to allow you to do so.
The point is – let your market decide the scale of your business, not the other way round. There is no point in creating a large business, with all the costs that involves, if there is no need. Alternatively, a small business may find it hard to service the clients you intent to work for, or the work you intend to do.
The size of your business will also determine the capital required to launch it. All things being equal its easier to launch a small business than a large one. However, it’s easier to launch a larger business now than it was.
If you have compelling answers to the first 3 questions (What sort of law firm do I want to own, who are my prospective clients, and why should these clients instruct me?), then it will soon be possible to attract external investment to law firms. This should allow legal entrepreneurs to create firms with greater scale, more quickly than was possible in the past.
For a topical example, Google Brilliant Law.