As you are going to have the wheel positioned between your legs it must not be to large so you can't reach the bottom bracket. For that you need to know your Inseam Length.
|minimum inseam for crank|
|ERTRO||Tire Width||Wheel Diameter||Typical Trail||Typical WB||145mm||155mm||170mm|
|16" big apple 50||345||50mm||445mm||-170mm||880mm||N/A||N/A||N/A|
|20" big apple 50||406||50mm||506mm||-140..-170mm||950..1200mm||N/A||N/A||N/A|
|24" big apple 50||507||50mm||607mm||-240mm||1140mm||79cm||80cm||81.5cm|
|26" big apple 50||559||50mm||659mm||-250..-300mm||1070..1380mm||81.5cm||82.5cm||84cm|
|27" big apple 50||-||50mm||-||-270..-310mm||1200..1380mm||?||?||?|
Another point to remember is the BB-FWA distance. When this gets below 40cms you will get in trouble when using a derailler. The chainline gets so extreme that the chain tends to fall off. You could get around that problem by:
- increasing the height of the BB, possibly increasing the seat height too, to have a comfortable seat-BB height difference (+15cm -> +25cm seems to be a good value).
- taking smaller cranks
- using smaller wheel sizes.