I'm exporting from lightroom, my files are exporting at around 3mb but I want them bigger, as big as the jpegs can be if possible. I've got the resolution at 300, and I've tried making it bigger and it makes no difference. Quality at 100 and I've also tried with this reduce.
Make sure to uncheck "Resize to Fit" and set the jpg quality to 100 and do not limit the file size. This will give you a jpg with the full resolution available from your image and minimum compression - the best you can get.
First, let me point out that F. McLion's answer is one that I agree with 100%
However, the idea that 3MB is not big enough and you want them bigger simply isn't a valid approach. The MB of the file does not tell you anything about how you can use the photos. The MB is misleading and meaningless, and 3MB photos may be perfect for your needs. You can be easily misled into taking unnecessary extra actions because you think the 3MB is a meaningful number.
You really want to look at the number of pixels (width and height) and this will tell you if the photo is useful for a certain purpose.