For the past decade, offshore oil and gas observed more challenging realms of the industry which include ultra-deep waters, harsher environments, and higher pressures and temperatures. The most notable of such is the challenges of high pressure high temperature (HPHT) conditions which are still regarded as uncharted territories and have prompted the industry to deliver safe and reliable solutions. Subsea design for HPHT conditions presents numerous engineering challenges which can limit conventional technology designs. For riser systems, HPHT conditions can require higher than usual pipe wall thickness, which can lead to pipe manufacturing and fabrication issues as well as riser payload and hang-off system challenges to the floating facility. Conventional design codes will be reviewed for riser sizing, hence suitable assumptions will be derived for riser sizing methodologies in HPHT applications. The purpose of this paper is to discuss the conventional thin-wall and thick-wall riser wall sizing designs and provide recommendations the appropriate sizing of the riser in HPHT applications.