One of best dimensions for smaller planted aquariums are the 15 / 20 gallon longs.
In Asia, one can find 60cm X 23 X 28 (35 litres/9+ gal) or similar. This tank size fits a 60cm light fixture exactly as well. This tank dimensions gives an amazing ratio for the smaller volume. Having more length for fish to traverse also makes the planted aquarium environment much more interesting. In the US, the 12 gallon long (35.4" x 8.3" x 9.4") is great but does not match with standard lights. The 20 gallon long fish tank measures 76.2 x 33 x 33 cm (30" x 13" x 13").
The 40 gallon Breeder is also a good entry level size dimension. The ratio of shorter height with more depth gives two advantages; it's easy to light up and has more depth for those serious about aquascaping. Choosing this over the smaller 20 gallon fish tank gives more options regarding planted tank plant & fish selection.
An ADA90p 90cm x 45 x 45 (47 gallon) by Jeff Miotke. A good sized tank for any serious aquascaper; large enough to give great choice in terms of aquatic plants, yet not so large to become extremely demanding in terms of hard scape & time.
For now, the ADA 120p (120 x 45 x 45 cm) commands what is widely considered the golden ratio in height + depth + length. It has enough volume for serious aquascaping, yet it is not as cavernous to fill with hardscape compared to slightly larger dimensions that push costs to escalate sharply. Many competition scapers in the IAPLC use this dimension. The picture of tank at the very top of the article is my mid 2017 ADA 120p.
Same length & height as above, but with more depth (120cm X 60 x 45). This complex scape by Roger Goh uses a deeper tank to give more room for creating structures & increased depth perception.