IS-L would be the answer...The Mark X (and the Mark II family before it) has the 'right' size for a sports sedan: bigger than the IS but smaller than the Crown/GS.
They're currently all closer together in size than you'd think. The 3rd-gen Lexus IS sits on a 2800 mm (110.2") wheelbase, while the current Mark X and 4th-gen Lexus GS share a 2850 mm (112.2") wheelbase, and the outgoing Crown Royal/Athlete use a slightly longer 2860 mm (112.6") wheelbase.
In overall length, the "stepladder' is as follows:
3IS: 4,665 mm (183.7 in)
Mark X: 4,750 mm (187.0 in)
4GS: 4,850 mm (190.9 in)
Crown14: 4,885 mm (192 in)
In width (excluding outside mirrors), Mark X and Crown are a bit narrower, a natural given their JDM/Asia orientation. Here are the numbers:
Mark X: 1,795 mm (70.7 in)
Crown14: 1,800 mm (71 in)
3IS: 1,810 mm (71.3 in)
4GS: 1,840 mm (72.4 in)
Enough of the present, though. Let's shift towards the future. The transitions from N/New N to TNGA-L platforms imply longer wheelbases (given the kinda/sorta front-mid-engine placement) combined with less overhangs to minimize increases in overall length. Based on the Crown Concept dimensions, the Crown14 (N platform) to Crown15 (TNGA-L) transition implies a 2.4" longer wheelbase and a 1.3" increase in overall length.
Stepping aside for a bit, let's review what's happening with the N-to-TNGA-L transitions. Lexus LS and Toyota Crown are the first ones we know about.
(Crown Athlete and Royal, like Lexus ES, seem to be on an inviolable, clockwork-like 4-to-6-year cycle). Lexus IS is a given, it's safe to say. Lexus' 5GS and Toyota's Mark X, however, seem to be in a state of flux and indecision where Toyota's not sure how to proceed. With all due respect, Mark X is not so much a smaller GS/Crown as it is a sportier, rebodied Crown alternative with reduced overhangs that sells in a different JDM Toyota dealer network from Crown. The current-generation Mark X has been around since October 2009 (2010 model year), with facelifts for the 2013 and 2017 model years. Also notable is that the Chinese market version of Mark X, the Toyota Reiz was a flop whose production ended last week, per
Wikipedia.
It's probable, as ssun30 suggests, that Lexus' 4IS will see yet another growth spurt vs its predecessor. Toyota, as stated earlier, is probably struggling with a decision on what to do with the next generations of Toyota Mark X and Lexus GS, large sedans in a crossover and SUV-mad world. Assuming they both survive, ssun30 might be on to something in suggesting that Mark X could align itself closer to IS and morph into something clearly smaller than Crown and Lexus GS. In that scenario, Toyota/Lexus TNGA-L sedan pairings would come in 3 sizes as follows:
Small: 3rd-gen Toyota Mark X / 4th-gen Lexus IS
Medium: 15th-gen Toyota Crown Athlete and Royal / 5th-gen Lexus GS
Large: 7th-gen Toyota Crown Majesta / 5th-gen Lexus LS