在AWS应用迁移管理器中，可以通过设置策略来限制资源的大小及数量。例如，下面的代码将限制AWS应用迁移管理器在迁移过程中启动的最大实例大小为t2.medium：

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "LimitInstanceSize",
            "Effect": "Deny",
            "Action": [
                "ec2:RunInstances"
            ],
            "Resource": [
                "arn:aws:ec2:*:*:instance/*"
            ],
            "Condition": {
                "ForAllValues:StringEquals": {
                    "ec2:InstanceType": [
                        "m5.large",
                        "m5a.large",
                        "m5d.large",
                        "m5ad.large",
                        "r5.large",
                        "r5a.large",
                        "r5d.large",
                        "r5ad.large",
                        "z1d.large",
                        "t3.large",
                        "t2.xlarge",
                        "t3.xlarge",
                        "m4.xlarge",
                        "r4.xlarge",
                        "c4.xlarge",
                        "c5.xlarge",
                        "t2.large",
                        "t2.medium"
                    ]
                }
            }
        }
    ]
}

这段代码中，Deny代表禁止，ec2:RunInstances代表运行实例，Resource中的Instance/*代表所有实例。而Condition中的ec2:InstanceType代表实例大小，其取值为m5.large等。由于设置了Deny的条件，所以条件内的实例大小都不能被启动。