You'll need to save the text as an unindexed property. To do this, you need to store the value explicitly as an array to specify excludeFromIndexes: true, eg.
var entity = {
key: datastore.key('Company'),
data: [
{
name: 'description',
value: "Some long text greater than 1500 bytes ...",
excludeFromIndexes: true
}
]
};