The purpose determines the method. For general usage, any enclosed space under the roof counts. The easy way is to measure outside, then deduct 5% for interior walls etc. which will be close enough. For loan/realtor/mortgage purposes they may want a more exact figure measured inside and may not want attatched garages included. One builder I worked for included porches to make the homes eligable for VA/FHA financing minimums but this is not normally done. For HVAC purposes just use the outside measurements, deducting any unconditioned space.
Most houses are roughly rectangular, and I use the "box method" when measuring them. Take the measurements of the main "box", then deduct the inset 'boxes' (like flush porches) from that or add the projecting 'boxes' (bump-outs and additions) to it. Makes it fast and easy. Deduct 5% for interior walls if needed.
Phil