What is the smallest baseboard size for OO/HO gauge?

Based on the principle of a layout configured as a loop the smallest practical baseboard size for an OO/HO will be 31 inches by 31 inches (about 80cm x 80cm). This value is based on a track of 'First' radius (371 mm/ 14.6") which is the smallest conventional option for a loop. A baseboard that is 31" square will ensure that a first radius loop will fit within the edges of the board without any overhang of the track.

Other schools of thought, including manufacturers, may recommend that the smallest practical loop would be based on a 'second' radius track segments (438 mm/ 17.2"). The reason for this is based on the principle that locomotives and rolling stock are not ideally configured to continually take the tightest curves and as such the chance of derailments are higher on the tightest turns, especially at higher speeds.

Those looking to experiment with 'flexible track' may find that option offers the opportunity to create a bespoke layout that might be tighter than first radius although the principle still remains that the tighter the turn the greater the chance of derailment and subsequent damage to the locomotive and rolling stock.