For family size of 1-2 people 6-10 liter water geyser is enough if you take bath via bucket.

If you guys take hot water bath via shower then 15L is enough.

A lot also depends on how long is underground pipe from geyser to tap? How harsh winter is in area you live? Buy one bigger size of water heater if you live in hilly areas of India.

Crompton Rapid Jet 3-L Instant Water Heater (Geyser) with Advanced 4 level Safety (White)
  • PRODUCT: Crompton's smart instant water heater with powerful heating element
  • TECHNICAL SPECIFICATIONS: Wattage: 3000 W; Capacity: 3L; Pressure 6.5 bar
  • 4 LEVEL SAFETY: Capillary Thermostat, automatic thermal cut-out, pressure release valve and fusible plug
  • ANTI-SIPHON PROTECTION: Designed to prevent any backflow of water and protect the heating element from dry heating damage
Orient Electric Aura Rapid Pro | 5.9 Litre Instant Water Heater | Stainless Steel Tank | Shockproof | 6.5 bar pressure compatibility | Suitable for low & mid rise buildings| 5 years warranty by Orient
  • INDIA’S FIRST 5.9L INSTANT WATER HEATER: India’s first 5.9L instant geyser, Aura Rapid Pro, is Orient Electric’s revolutionary launch.
  • LONG LIFESPAN WITH STAINLESS STEEL TANK: Made with high-grade Stainless-Steel tank, this water heater comes with an extended lifespan, fulfilling your hot water needs endlessly.
  • ENJOY FASTER HEATING AND OPTIMAL HOT WATER: The electric water heater comes with a heavy Copper heating element that heats water faster, fulfilling your hot water needs anytime.
  • SHOCK-PROOF AND RUST-RESISTANT BODY: The high-strength polymer body of this water heater is shock-proof and rust-resistant for ultimate safety and extended lifespan.
  • SUITABLE FOR LOW-RISE AND MID-RISE BUILDINGS: With a pressure compatibility of up to 6.5 bars, the instant geyser is suitable for low-rise and mid-rise buildings.
AO Smith Geyser 15 Litre 5 Star Rating (BEE) | Powerful 2KW Heating | Storage Water Heater With 2X Corrosion Resistant Blue Diamond Glass Tank | Warranty: 5 Yr Tank, 2 Yr Comprehensive | HSE-SHS-015
  • DOUBLE PROTECTION FOR SAFETY: Enjoy ultimate peace of mind with this AO Smith 15-litre water geyser. It features a factory-set thermostat, multi-function safety valve, and thermal cut-out, ensuring safe and reliable hot water for your bathroom every day.
  • DURABLE & ELEGANT DESIGN: This AO Smith 15-liter water geyser is crafted with a glossy ABS plastic body and blue diamond glass-lined tank for 2X corrosion resistance, making it the best 15L storage water heater for style and durability.
  • ENERGY-EFFICIENT 5-STAR GEYSER: Save on electricity bills with this BEE 5-star rated 15-litre geyser. Perfect for eco-conscious homes, this water heater ensures efficient heating while reducing energy consumption, making it a top choice in India.
  • BUILT FOR HIGH-PRESSURE USE: This AO Smith 15-liter storage water geyser can handle up to 8 bars of pressure, making it ideal for high-rise apartments or hard water areas. Enjoy uninterrupted hot showers without compromising performance.
  • LONG-LASTING WARRANTY COVERAGE: With a 5-year warranty by AO Smith on the inner tank, 2 years on the glass-coated heating element, and 2 years comprehensive by AO Smith, this AO Smith 15-litre water heater offers unparalleled after-sales support for peace of mind.
Longway Superb 10 Ltr 5 Star Rated Automatic Instant Water Heater for Home, Water Geyser, Electric Geyser with Multiple Safety System & Anti-Rust Coating (Gray, 10 Ltr)
  • Type: Instant, Capacity: 10 Liters, Wattage: 2000 Watts, Pressure: 8 Bars, BEE Rating: 5 Star Rated Energy Efficient Geyser, Inner Tank Material: Stainless Steel (SS) Tank with special Anti-Rust coating - 2X Corrosion Resistance; Outer Body Material: Metal (CRCA).
  • Highest Saving: BEE 5-star rated with Advanced heat resistant Technology for Power Savings. Perfect if you want to go green. Quality check: 5 safety functions and 15 Quality checks by a team of professionals to ensure the best product for you. Extra heavy duty element for extra heating & durability.
  • Product Features: Superior polymer coated tank, nano poly bond technology for excellent corrosion resistance and oxidation resistance even in high temperature and pressure 100% ISI Marked & BIS approved Copper Heating Element to ensure powerful heating.
  • Package Content: Electric Geyser, Instruction Manual.
  • Warranty Period: 1 Year Limited Domestic Brand Warranty on Product & 5 Years on Inner Tank/Container. Enjoy the hassle free warranty by Longway on your product.

If your bathroom is on top floor of the house or an open bathroom? Here I recommend gas geyser as these need to be installed in open only. However buy only from renowned brand which can provide you service every year. Gas geyser need special care and maintenance. A yearly service from authorized service guy once a year at start of winter is enough for gas geysers.