SAN FRANCISCO — When Apple wants to make a big splash, it returns to its history. Thirty years ago at the Flint Center for the Performing Arts, a roomy auditorium in Cupertino, Calif., Steven P. Jobs introduced the original Macintosh. On Tuesday, Apple will come back to the center to unveil a set of long-anticipated products: two iPhones with larger screens, and a wearable computer that the media has nicknamed the iWatch. The so-called smartwatch will be the first brand-new product unveiled under Apple’s new chief, Timothy D. Cook, who took the helm after Mr. Jobs died nearly three years ago. It is expected to come in two sizes and combine functions like health and fitness monitoring with mobile computing tasks like displaying maps, said people knowledgeable about the product. It will have a unique, flexible screen and, like the new phones, will support technology that allows people to pay for things wirelessly. “I believe it’s going to be historic,” said Tim Bajarin, a consumer technology analyst for Creative Strategies who attended the original Mac event in 1984. He added about the much-anticipated Apple watch: “The design of this product is all Tim’s fingerprints.” With its first wearable computer, Apple will enter a growing market for fitness-tracking accessories and so-called smartwatches filled with gadgets from Samsung Electronics, Fitbit and Nike. And with the two larger phones, the iPhone will fight back against Samsung, whose big-screen Galaxy smartphones have wrestled sales away from Apple over the last few years. While the iPhones are expected to be released in the coming weeks, the watch is unlikely to be in stores until next year, several people said. The price of the new devices is not yet clear. Apple, which is highly secretive, has not officially commented on any of the new products. But multiple employees for Apple and its partners who were briefed on the products shared some details on the condition that they not be identified. Some said the smartwatch was one of Apple’s most ambitious projects to date. The company put an enormous amount of time and money in the wearable device’s sensors so that they would track movements and vital signs, like heart rate and footsteps, much more accurately than existing fitness devices, two employees said. It has a flexible display panel that is protected by a cover composed of sapphire, a type of tougher glass, they said. The device’s circuit board, which includes its sensors and chips, was described as tiny, about the size of a postage stamp. For replenishing the battery, the smartwatch will rely on a wireless charging method. Apple had at one point tested solar charging for the watch, but that experiment failed. For software, the watch will take advantage of HealthKit, a set of tools for storing health data that Apple introduced in June. The device will also rely heavily on Handoff, a new software tool that makes it easy to share content from one Apple device to another. The bigger iPhones, which have been widely written about over the last year, will come in two sizes, one measuring 4.7 diagonal inches and the other 5.5 diagonal inches. The larger version will be the more expensive model. Unlike the older iPhones, which have somewhat sharp edges, the new ones have softer, tapered edges, similar to the latest iPads. Continue reading the main story Continue reading the main story Continue reading the main story To deal with concerns that a bigger phone will make typing with one hand difficult (the current iPhone has a 4-inch screen), some changes to the design of the iPhones’ software interface will allow people to type or use apps with just one hand — there will be a one-handed mode that can be switched on and off, two employees said. The wearable device and the smartphones will include hardware and software that supports technology called near-field communication. That allows devices to exchange information wirelessly over very short distances, which could make paying for things with a phone less of a hassle. Apple has teamed up with Visa, American Express and MasterCard to support the payments system, said several people involved in the partnerships. In the deal, these people said, iPhone owners will be able to use their devices as a sort of digital wallet, giving customers the ability to pay for items at select partner merchants without handing over cash or a credit card.