17650 - 18650 - 26650
17650 and 18650 Li-ion batteries are rated at 3.7 volts. 17650 is roughly the size of two CR123A batteries where the 18650 is slightly larger in diameter (roughly the same length) and the more commonly selected replacement. Use of a single 17650 or 18650 to replace the use of two CR123A or two 16340 (RCR123A) batteries is perfectly acceptable and is often beneficial since the 17650 or 18650 has a higher total capacity (mAh) compared to the two individual 16340 or CR123A batteries.
26650 is about 30% longer than a C Cell battery. The 26650 offers the highest capacity (and thus run times) due to the size of the battery.