Just create a file under your lib
folder and use the documentation here: http://docs.telescopeapp.org/docs/custom-fields to remove or add fields to that Schema.
EDIT: Sorry, but reading carefully your comment I understood you want to modify the autoForm props rather than the Schema itself. To change the maximum allowed value, do something along these lines:
Posts.removeField("body");
Posts.addField({
fieldName: 'body',
fieldSchema: {
type: String,
optional: true,
max: 5000,
editableBy: ["member", "admin"],
autoform: {
placeholder: 'Cannot exceed the maximum length of 5000 characters',
row: 10,
type: 'textarea'
}
}
});