Estimatiing Backup Space for Agent for Windows. How much space will I need?

If you ensure to install the CBT (Change Block Tracking) driver the incrementals will be small when it runs after the initial full.

This explains retention - Short-Term Retention Policy - Veeam Agent Management Guide

Check out the official Veeam Calculator. You should be able to punch in your requirements and it will tell you how much repository capacity you’ll need.

Thanks.

What is the CBT and where would I find it to install it?

If its managed by VBR, then from console, in Infrastructure tab.

If its not managed by VBR

Double-click the Veeam Agent for Microsoft Windows icon in the system tray, or right-click the Veeam Agent for Microsoft Windows icon in the system tray and select Control Panel.

From the main menu, select Settings.

Click Install CBT driver.

You can check here for instructions on how to install the Veeam CBT Driver. Note that this is not required for incremental runs, it is just considered to be more efficient way for Veeam to keep track of changed blocks, which could decrease processing time.

The CBT driver is optional for the Windows agent and does only speed up your incremental backups. They will not be any smaller with the CBT driver as you will always do incrementals following the first full (unless you set something else). 

The more important question for you is your change rate. If you have e.g. a change rate of 10% you will add 10% of your full every day adding up to your backup storage needs. It seems you have poor compression for your backups (900->800GB). Try to generate another backup in the chain, which will be an increment. The size of it determines you incremental size. Now do the maths: how many restore points do you want to keep? Lets say 14 days with this 10% change rate → 800GB + (13 * 800GB*10%). 13, as the first day is the full. This will leave you with a space consumption of roughly 1,8TB.

Best,

Michael