There are a few steps to consider when modding a tech tree
1st) To resize research tree windows, you need to go into the Player.entity file. That data to edit is about 3/4 of the way down the entity file (id recommened getting notepad++, its great for editing those files). Basically the size is in matrix format [horizontal position (distance from left of screen), vertical position (distance from top of screen), window width, window height]. Be sure to add your additional research entities to the counters in both the player and entity manifest files!
-the above is useful if your adding tech's but you start running out of room, you can move the windows around so you can get more space!
2nd) Editing research files, like the others have said, simply requires editing the respective entity file, if you are creating a new file you will need to reference the name of the research entity in BOTH the player.entity, and the entity.manifest files. Editing an existing or new research file is straightforward, each tech will have some entity modifier or research float modifier that puts permanent, non-stacking effects on other entities (like ships, structures, and planets). Check the modding forum here for a list of all the research modifiers at your disposal!
3rd) Editing names of research files, you won't want to stick with vanillna research names so you would simply edit the "NAME" and "DESCRIPTION" files in the research entity to a new string (i.e. "IDS_RESEARCHSUBJECT_ARMORNEW_NAME" and "IDS_RESEARCHSUBJECT_ARMORNEW_DESCRIPTION"), then the next step is too ADD those, to the english.str file (that has all the strings in it). It is easiest just to copy an existing pair of name/description strings and edit them. Then once you have your new strings, make sure to add your new strings to the total count at the top of the page!
There are more specifics involved, but I hope that helps!