I want to replace my phone's system typeface with adobe's Source Sans Pro, but it's too small for my eyes. How can I resize it? Thanks in advance?

Which phone?

I'm using an Android phone. I rooted it and I've been replacing the system fonts with fonts I like. I really like source sans pro, but it's just too small.

(Can’t help in that case, but added it to the topic title for clarity.)

Is it easy to resize ttf files? Source Sans Pro is an open source typeface. All I need is some method to make it bigger, and then I can put it on my phone.

Well, any font editor can do it, but fiddling with all the values is tricky. 

I would suggest to try the Fontsquirrel webfont generator:
https://www.fontsquirrel.com/tools/webfont-generator

You can upload a TTF font there, change the percentage of “x-height matching” and download the result. That’s pretty easy.