Playing SAYC, you open 1 Minor. Left passes and partner bids 1 Major. How do you figure out if partner has 4 carder major or 5 carder?
Obviously if left overcalls, then partner can use a negative double to indicate he has a 4 carder or bid 1 Major to show a 5 carder. But otherwise, is there any convention to figure this out?
In general no, there is no convention to convey this information because it is not (yet) important enough to warrant co-opting a call sequence. The subsequent auction will enable the partners to first determine if an 8-card fit exists, then whether either notrump is a suitable contract, or notrump is unsuitable but only 7-card fits are available.
Opener will show 4-card support eagerly (either by an immediate raise or a delayed jump raise) so that Responder will know there is an 8-card fit.
If forced by Responder to make a preference then Opener's minimum return to Responder's will be assumed to show only 2 card support. This will often be passed unless Responder has unrevealed strength to compensate for the lack of fit
If Opener returns freely to Responder's first suit then this delayed support shows three cards. At this point Responder holding 5 can indicate this by passing, inviting game, or jumping to game depending on his knowledge of the combined partnership assets. If Responder holds only 4 cards then a choice must be made to play in a Moysian fit of the major; in notrump; or in some other suit.
An essential point to note in these auctions is that the beginner's rush to rebid a 5-card suit is a terrible habit, and to be avoided whenever possible. Fast rebids of a suit should be reserved for showing 6-card holdings, so that partner can freely raise to show 2, revealing the 8-card fit.
AN exception to the above exists when Opener rebids 1 Notrump. Many partnerships will play some variation on Check-Back Stayman or New Minor Forcing to then explore major suit fits in more detail.
Update - as noted below in a comment:
by "fast rebid" is meant "at the next round of bidding." The tempo of the rebid itself, slow or fast, cannot legally (or ethically) be taken into account by partner!
