Is there any down side to over sizing the boiler if you have excess thermal capacity?
Generally, "no," but I think moderation applies here as to most things. My system is over-sized: 140,000 btuh boiler, 1000 gal pressurized storage, shop calculated heat loss = 35,000 btuh at -35F, highest measured heat loss so far = 18,000 btuh in a 24 hr period.
Advantages: can charge storage from 120 to 190F in about 4-6 hours while also meeting demand and then meet demand from storage; need to burn every other day at the most, less often with mild outdoor temps; with a little care on loading (weighed wood burns makes this easy) boiler can burn full out and coast to fire out while bringing storage up to 190F and no idling.
Disadvantages: need to carefully plan plumbing (pipe size, circ gpm, pump head, etc.) to insure that system can deliver maximum output from boiler to storage and/or system; requires larger pipe sizes $$$ than a smaller boiler; need sufficient storage (thermal capacity) to accept boiler output.
If storage is intended to meet system demand between boiler firings, then you need to determine your heat loss btuh and size storage to be able to meet this demand during the times between firing your boiler. Example: if "normal" heat loss is 40,000 btuh and you want to fire once per day, then you need 40,000 x 24 = 960,000 btu available from the boiler/storage. Assume that your burn period is 4 hours, therefore boiler will meet demand for 4 hours and storage will need to meet demand for 20 hours. Needed btu's in storage at end of burn = 20 x 40,000 = 800,000.
If you need 140F water to meet your system demand, and you can heat storage to 190F, then you need about 2000 gal of storage to meet demand solely from storage (2000 x 8.34 x 50 = 834,100). If your system require hotter water, needed storage will increase, and vice versa. If you want storage to meet demand for a longer period, the storage will increase, and vice versa.
Incidentally, 800,000 btu's will require about 200 lbs of wood per burn (960,000 / 6,040 btu-lb / 0.8 efficiency). Efficiency will increase if the boiler/storage is in the space to be heated and efficiency may decrease if not. Btu's per lb of wood is based on 20% MC and 400F stack temp.