How large were the lands of Dukes, Earls, Barons, Lords? How much people would they usually have? If one plot of land grows to big would they split it in half and have another Lord take rule of it?
"Lord" isn't a discrete title, but an umbrella term for any member of the nobility who held land. A knight who held a single village with a population of 20 was as much a lord as the Duke of Northumberland. I'm curious what you mean by "grow the number of peasants under him"; it's not like you can really farm people (well, outside dystopian novels anyway) so I assume you mean by means of trying to attract additional people onto his lands from neighbouring regions, perhaps by means of lower rents or higher status. Of course, without vacant land to fulfil those tenancies, our lord is going to run into problems.
This is where the Marcher Lords have a natural geographical advantage. Robert of Rhuddlan is a particularly pertinent example of the situation you posited: Robert was a cousin of Hugh d'Avranche, Earl of Chester, and served as a marshall of his forces, establishing himself as Lord of Rhuddlan after capturing and fortifying land around Twthill the early 1070s. Although still a vassal of Hugh d'Avranche, Robert raised an army and expanded his lands aggressively, intervening in a number of wars between Welsh kingdoms and civil wars, and using any opportunity to seize and fortify land so that by the time of Domesday Book in 1086, Robert held essentially all of North Wales. He's listed 'in Domesday Book as tenant-in-chief of those lands, meaning he held them directly from the king, and could therefore be considered "Lord of North Wales", although his original holdings in Rhuddlan were still held from Hugh d'Avranche. Interestingly, after Robert's fairly sudden and ignominious death, all of his lands reverted to Hugh d'Avranche, at least until the Welsh revolt of 1093.
Robert's aggressive campaign of expansion and conquest would have been a prime recruitment tool, and indeed his 'warband' was thought at times to be at least as big as the army of King William. Newly-conquered territory was a prime means of recruiting a population, as it attracted in particular smallholders who were seeking to hold much bigger areas of land.