All conventional redundant RAID systems require identical-size disks. If you don't use same-size disks, the extra space is not included in the RAID array. If you want to increase the size of a RAID 1 array, you'll have to buy two new disks.
The MS Forum thread you link is describing WHS without Folder Redundancy enabled. WHS offers one-click Shared Folder Redundancy. If you enable this on a shared data folder, ALL files in that folder are duplicated on a different disk. If redundancy is enabled, you'd have to lose more than one disk to lose ANY files in that folder.
The other "lost files" mentioned are configuration data from the System disk. It's a pretty minor deal to reconfigure WHS remote access and user accounts if the System disk is lost. A lost or corrupted System disk will require a "re-installation" of the WHS server. This is done via a system recovery option in the WHS installation menu.
If Folder Redundancy is disabled, then WHS is like any other file server without disk redundancy enabled: You can lose data if a disk fails.
Since you have Acronis licenses for all your PCs and are willing to use RAID 1 on your file server, then you do have many of the features offered by WHS. Some differences include:
1) Addition of new PCs requires the purchase of additional Acronis licenses. WHS includes licenses for ten PCs.
2) WHS sets up backups automatically with the installation of a small client connector. Installation and configuration of Acronis is more complex
3) WHS warns everyone in the office if any PC is missing current AV software, is missing a firewall, if backups are failing on a PC, or if there's a problem with the server. With Acronis, you have to rely on the individual PC user to do something about failing backups.
4) As noted, any number of disks of any size and any type can be added transparently to the WHS shared folders. If you add additional disks to a conventional file server, they will normally end up as another drive letter.
5) WHS allows menu-based remote access to any client PC if the client is a business version of Windows. Without that feature, you'll need to program port forwarding for every PC and change the Remote Desktop ports on each. Or you'll need to install third-party software for remote access.
6) WHS includes built-in password-protected web sites for remote access to file shares. With desktop-based file servers, you'll need to install and configure web server software or FTP software.
7) WHS backups de-duplicate common files on all PCs in the network. If all PCs are running the same version of Windows or MS office, for instance, only one copy of Windows and Office will be backed up. This minimizes the size of the backups. Acronis will back up every file on every PC, making the total backups larger.
8) WHS automatically manages backup history for all PCs. Older backups versions are removed on a schedule selected by you. This means that total backup sizes usually don't grow significantly after a few months, but you still have backups stretching back several months, with an emphasis on more recent backups. I haven't used Acronis lately, so I don't know how it manages backup size or automated removal of older backups.
9) WHS keeps a copy of all current and previous drivers for all client PCs in a single place. If you have uncommon disk controllers or NICs, this can come in handy both for a full restore and for replacing a single driver.