What Free Tier Provides
Free tier gets 30GB of EBS storage, plus a lot of other things, including:
- A single t2.micro EC2 instance
- An elastic load balancer
- 5GB of EFS storage
- An RDS relational database instance, with 20GB of storage and a set number of IOPS
- DynamoDB no-sql database, 25GB plus read/write capacity
- Lambda serverless computing
Plus a whole host of other services. You can see the list here.
What is Free Tier For
The free tier seems to be designed to let you run single instance, with associated services, to help you work out if AWS is right for your workload. I guess they're also trying to get you hooked so you start paying once the year runs out.
Staying Within Free Tier
The best way to stay within this 30GB is to allocate 30GB or less to your EBS volumes. For example, two 15GB volumes, or one 10GB and one 20GB. Personally I would leave a small buffer if you really want to avoid being charged - perhaps try to use less than 29GB. AWS often use GiB, which is different from GB, but the free tier does say GB. A GiB is larger than a GB.
You can use CloudWatch to monitor your resource usage.
Operating Systems and EBS Disk Space
You said Windows server is 19GB, which fits within your free tier. Linux images are significantly smaller than Windows. Since you can only run one instance for free, and each is less than 30GB, this seems fine.
If you want to run two instances you obviously just need to keep the disk space down. With Windows you can do things like use minimal swap space, disable things like hibernation, uninstall components you don't need, etc.
Though you can resize the EBS volume, reducing the size looks problematic, if it's possible at all. I initially suggested creating a large volume, optimizing windows, then reducing the size, but as pointed out by Michael I think you should start with the volume size you need.
Since you have 5GB of EFS included you can consider putting some of your data here. Note that once free tier finishes EBS costs $0.10 per GB but EFS is $0.30 per GB. EBS resizes in a single AZ (data centre), EFS stores data in multiple AZs (data centers). EFS scales to very high throughput as well.
Will likely still have an AWS bill if you use services outside the free tier. For example you will likely want to take EBS snapshots, which are volume backups. You get 1GB free, but if you use 30GB of disk your compressed snapshots will be significantly more than 1GB.