Having that many assets, especially if a lot of them are windows, you will want to turn off windows event log scanning, or at least limit it to 1 to 2 days. You will want to use MSSQL full version of some sort - if your teams will rely on it, or ITSM like ServiceNow, etc... you might want to consider an Always-On SQL cluster or something. Anyways, look over the database size articles as far as which tables grow large - many do not, but the event log history and a few other history tables surely do. You will need multiple scan servers.. a lot of them probably, as you set the number of simultaneous scans for windows and network items... so it will never catch up from the queue unless you segment off the load across multiple scan servers. Fortunately those are easy to get going. Definitely use IIS full version and not IIS Express. You will have to be creative when dealing with that many subnets, exclusions, etc just due to the sheer numbers - so what I do is run SSIS packages, or API to update/import from various other systems, like an IPAM system so you don't have to manually create or maintain all of those IP Locations, for example. Stuff like that is not supported, but it's a necessity - fortunately its MSSQL behind the scenes, so become best friends with a DBA and the sky's the limit.
OH. Make sure you have a dev/test environment/instance - it's covered/allowed in the License Agreement Terms, and it's a MUST due to the fact that some bugs pop up here and there with new versions, that can actually cripple a system that large if you're not careful. Make sure to do upgrades in test first, and test the agents, etc... and closely watch the forums and changelog page for Lansweeper.
Specs on SQL server isn't as high as you would think - I'd say like 4p4v if even that... and 32GB of RAM or so... but you will want to work with support/DBA to change the default location of the databases, which is on the C: drive. The installer sets up the database automagically. Scan servers don't take up much at all - standard 2p2v with 8GB of RAM and standard boot drive size. It's just a windows service. the main server depends on how many users will be accessing the website - but standard 2p2v with 16 to 32GB of RAM probably... and standard disk size.