If you want to keep your AWS usage within the free tier, you need to stick to t1.micro instance type. Juju defaults to a larger instance type by default to more easily support multiple, large Jujus models.
Adding the instance-type constraint will instruct Juju to use a smaller instance type than its default for its controller (via --bootstrap-constraints) and any models (via --constraints).
juju bootstrap aws aws \
--bootstrap-constraints 'instance-type=t2.micro' \
--constraints 'instance-type=t2.micro'`
If you would like to learn more about which constraints that Juju supports, we have thorough documentation.