EnterraHost
This is the first decision most people make and the one they most often get wrong, usually in the same direction. Mailbox size feels like the thing to buy generously, and it is the easiest thing here to over-buy and the easiest to fix later.
What a gigabyte of mail actually is
Modern mail is small. A text message is a few kilobytes, and most of what fills a mailbox is attachments rather than conversation. A gigabyte is roughly five thousand ordinary business messages, or a few hundred messages with substantial attachments.
That puts 10 GB at somewhere in the region of fifty thousand messages. For one person reading and answering email, that is years. Most people who assume they need the larger size have never come close to filling the smaller one, because the mailbox they are comparing against is a free account they have had for fifteen years.
The exception is real, though. If your mailbox is where contracts, drawings, photographs or spreadsheets arrive and stay, the arithmetic changes, and it changes fast. A single mailbox that receives large attachments weekly will outgrow 10 GB in a couple of years while a colleague in the same office never fills a tenth of it.
The question to ask is not how much you have now
It is what arrives and stays. Three things push a mailbox up faster than people expect.
The first is attachments that live in the mailbox rather than in a shared drive. If several people need the same file, mail is the wrong place for it and the copies multiply.
The second is a mailbox that several people use. A shared address like info@ on a real mailbox collects everything, and it grows at the rate of the whole team rather than one person. That is an argument for 50 GB, or for a distribution list that forwards rather than one mailbox everybody logs into.
The third is years of history on the same address with no cleanup ever done. That is not wrong, but it is a choice, and it is what makes 50 GB the honest answer for an established business.
Why starting smaller is usually right
Storage is added later in blocks, without rebuilding the mailbox, and adding storage to a mailbox takes minutes rather than a migration. Nothing about choosing 10 GB commits you to 10 GB.
That changes the shape of the decision. Buying the larger size up front is paying now for space you may use in three years. Starting small and adding a block when you need it costs the same per gigabyte and only starts when you actually need it.
The one thing to avoid is running out and not noticing. A full mailbox stops accepting mail, and the first sign is usually a bounce someone else reports rather than anything you see. Watching the storage once a quarter is enough, and it is the whole of the maintenance this needs.
Size and tier are separate decisions
Both sizes are available on both tiers, so storage never decides which tier you buy. A 10 GB Pro mailbox and a 50 GB Lite mailbox are both ordinary orders, and mixing them under one domain is normal.
Buy the tier for what the mailbox does, and the size for what it holds. If you are still choosing the tier, creating your first mailbox sets out what separates Lite from Pro. The prices for both sizes are on the business email page.
A reasonable default
If you have no strong reason to think otherwise, start with 10 GB. It is the cheaper choice, it is enough for most single users for years, and the upgrade path is a few minutes of work rather than a project.
Choose 50 GB when the mailbox is shared, when large attachments genuinely arrive and stay, or when you are moving years of existing history onto it and want the headroom from the start. Those are all cases where you already know the answer rather than guessing at it.