Perhaps a misnomer, the Sejm of the Kingdom of Poland does not simply represent Poland proper (which consists of Poland and East Galicia). Instead, it refers to the union of Poland-proper, Lithuania, and Livonia. Instead, Poland-proper is legally & colloquially referred to as 'Korona,' with its own Sejm (the Sejm of Korona). The Sejm of the Kingdom of Poland allows for representation from Lithuania, Korona, and Livonia all at once. That is, Korona is on equal political status as Livonia and Lithuania, with the Sejm of the Kingdom of Poland representing all three together as a national entity.
Initially, as with all Napoleonic constitutions, the constitution of Poland established the executive (in this case, the King of Poland) as substantially more powerful than his legislature (in this case, the Sejm). Furthermore, it made provisions for a bicameral Sejm, in which the upper house was entirely appointed. Later, with the expulsion of the House of Wettin and the implementation of a natively Polish noble family as Kings of Poland (The Poniatowskis), the constitution was revised to shift the balance of power towards the legislature, and particularly towards the Marszałek Sejmu, i.e. the Speaker. This revised constitution would also expand suffrage to all ethnic Poles & Lithuanians (though not 'Ruthenians' or Germans), whereas prior land-ownership had been a requirement.
The Bicameral Sejm
The Sejm is bicameral, with two houses - the Senate & the House of Deputies. The former - the Senate - is comprised of the Prefects of the Departments. That is, the governors of the provinces. They are joined by ten (Catholic) Bishops. The Senate, however, has virtually no power other than oversight, keeping the other houses in check with a veto (that can be overruled by the King of Poland), as well as broader powers to instigate an audit, demand written explanation of legislation, etc.
The latter - the House of Deputies - is elected by Sejmiks, or local assemblies, based in the various Departments. Each Sejmik sends a certain number of Delegates proportional to its population (that is, sejmiks in departments with bigger populations necessarily send more Delegates) and these Delegates are elected by Single Transferable Vote every four years, with no term limits. Each sejmik - as well as department - is named after the largest city within its boundaries.
The Polish Crown Lands - 'Korona'
The Duchy of Livonia
The Grand Duchy of Lithuania
(HRM) The King of Poland
The Duke of Livonia
The Grand Duke of Lithuania
Delegates to the Sejm