The two common rules
Fixed fraction risks the same percentage of the account on each position, usually somewhere between 1% and 5%. It is simple, it survives a bad run, and it ignores how good the opportunity is.
Kelly sizing scales with edge: the larger the gap between your probability and the price, the larger the stake. Full Kelly maximizes long-run growth and produces swings most people cannot hold, which is why half-Kelly and quarter-Kelly are the practical versions.
What prediction markets add
The loss is total and discrete. A binary contract does not drift down. It settles at zero, so no stop-loss reliably limits the damage on a position held to resolution.
Correlation is the other trap. Five positions that all depend on one election night are effectively one position, and any sizing rule applied per contract will understate the real exposure.
Where 0xinsider uses position sizing
0xinsider computes a Kelly fraction and a VaR for each wallet; both are in the Pro data export. On the flow side, seeing what fraction of a trader's book went into one market is usually more informative than the dollar size of the trade.
