No agent is required, but Alert Logic® does require one m3.medium instance per VPC within the desired scope that is set.
An appliance will be spun up in each in-scope VPC when scans are needed. If there are no scannable hosts, there will be no scanner. If no hosts are scannable because they've all been scanned recently, there will be no scanner.
The size of the scanner will be m3.medium, as long as it is available in the Availability Zone. If not, c5.large or c4.large are used, in that order. Alert Logic generally chooses the cheapest instance wtih dedicated CPUs.