For an integrated spreadsheet of the irregular verbs and long, mixed and short conjugations in past, present and future, see the Chart.
NB: The conjugation of any verb does not change even when prefixed or suffixed. If the verb being built upon is long, it will remain long. It will still follow the same rules as all other verbs in its conjugation.
So, if you have padomāt (to think over) or iedomāties (to imagine), they are both long because they are both built upon domāt.
Check out the pages on individual conjugations for a more in-depth examination of how the verbs are conjugated throughout various tenses.