Replying to Photo iamge sizing...confused

my 18MP SLR takes photos at nearly 5100x3100 (not sure on specific size but it is within 100pixels of both of those numbers).

and i import them to their specific folder depending on if it is of people, a trip, skiing, etc...but i was wondering if there is a way to resize them all together instead of editing one in post, then saving/exporting it and making the image size smaller. i use lightroom currently.

also, i have my camera set to conintuous file numbering (such as IMG_xxxx.jpg) etc.. but after i reformat my SD card it obviously starts back at _0001.jpg. and i already have certain files with that same name ina  folder, is there a way to override that or make the image name longer? i noticed EOS utility for my Canon also has the option to rename each group f images im importing to my computer and i can change the prefix/numbering on it too but i was curious if there is an easier way to keep them more organized. if any of you could share briefly what you do, i would apprecaite it and ++ you so i can get some better ideas on how to organize all my photos and make this easier.

cheers!