There is no specific way to achieve what you're asking without editing the source, however there are a few work-arounds which can get close to what you're after.
Firstly, in OpenTTD, some Towns are designated Cities. Cities start off with a higher population count and also grow faster compared to towns. You can disable the generation of Cities by changing the "Proportion of towns that will become cities" option in Advanced Settings to 0. Whilst there will still be some population variation between towns, the difference will be drastically reduced.
However, this will still leave a wide variation in population sizes between towns, with populations ranging from below 100 to above 1,000. Towns also come in 3 variants - small, medium and large. Whilst there is no way to influence the type of town generated when playing a new game, another work around exists using the scenario generator. Enter the scenario generator and have it generate terrain for you to your liking, just as you would have done when starting a new game. Once the terrain is generated, open the Town Generation tool. Select the size of town you wish to add to the map, Small, Medium or Large, and then click the Random Town button repeatedly until the desired number of towns has been generated. Save the scenario and return to the main menu. You can now use the Play Scenario option to play your new custom scenario.