In Commander, if a card refers to your hand size and you have no maximum hand size due to a card effect, how does that interaction resolve?

By default, a player's maximum hand size is seven (CR 402.2). Certain cards and effects can raise, lower, or remove this limit entirely. When an effect says you have 'no maximum hand size,' your hand size limit is considered to be infinite (or more precisely, there is no number to reference — CR 402.3).

When a card's effect references your 'maximum hand size' as a numerical value, and that value is currently 'none' (infinity), the result depends on how the effect uses that number. If it asks you to draw cards equal to your maximum hand size, drawing an infinite number of cards is impossible and the game cannot execute that instruction — this is generally interpreted as the effect doing nothing or being unable to resolve that portion (CR 107.1b covers the handling of undefined or inapplicable numbers). If the card instead asks you to discard down to your maximum hand size, having no maximum means you never need to discard, so the effect simply doesn't apply.

A concrete example: Suppose you control Reliquary Tower (giving you no maximum hand size) and someone plays a card that says 'each player discards cards until their hand size equals their maximum hand size.' Because you have no maximum hand size, you are unaffected by that instruction and discard nothing, while other players must discard normally.

It's worth noting that effects which set your maximum hand size to a specific number override the 'no maximum' effect depending on timestamp and layer rules (CR 613). The most recently applied effect in the same layer wins, so if Reliquary Tower removes your maximum and then another effect sets it to five, you'd have a max of five cards again.

In Commander specifically, there are no additional format rules modifying this interaction — the Comprehensive Rules govern it fully. When in doubt, treat 'no maximum hand size' as infinity and evaluate whether the referencing card's effect can function with an infinite value; if it cannot produce a meaningful game action, that portion of the effect does nothing.