GEG Controller described airspace incursion event when during very light traffic an IFR altitude was assigned above the delegated airspace.

Narrative

Aircraft X departed SFF in VFR conditions on a VFR flight following flight plan. Around 8;500 FT MSL; Aircraft X asked for an IFR clearance to same destination requesting 12;000 FT as a final altitude. I cleared the aircraft via direct and told him to climb and maintain one two thousand; which is the top of our airspace. Immediately after this; the pilot came back and requested 16;000 as a final altitude. Not thinking it through; I responded; 'roger; climb and maintain one six thousand'; which violates our LOA with ZSE. I completed a hand off with ZSE and transferred communication after amending the flight plan in the FDIO (Flight Data Input/Output). About 30 seconds later; as the aircraft was climbing through 12;500 FT; I realized my mistake and immediately called ZSE to advise them that the aircraft had already been cleared up to 16;000. The Controller responded with 'Roger' and gave his operating initials. There was only one other aircraft on my frequency at the time so the only causal to this event was human error. It was a slow day and traffic was not an issue. I had my RADAR filter limits set to FL240; so I know there were no conflicts; and in my opinion safety was not compromised; but it was an airspace violation none the less. Recommendation; the only thing that would have helped mitigate this incident is more attention to detail and a heightened alertness. It was a slow day and I think that these are the most dangerous times because you can let your guard down and make stupid mistakes such as this.