Hiya Bren. Pushups and dumbbell curls are only two exercises of many many others out there.
Basic info - 8-12 reps when starting off. Later on when you know how to use proper form, and know what your doing as there's less chance of injuring yourself - 6-12 for hypertrophy(increase in muscle size).. 4-6 for strength.
High reps are NOT for bulk.. they are for endurance. I have to disagree with MMA's post, the abs are just like any other muscle, if you want to increase their size (so that they are easier to see, better defined) you must stick to the same rules that apply to every other body part, which means adding weight to your exercises.
Also strength IS relative to size, if you want good size you need strength.
Seeing as you only have a set of dumbbells, here's a workout that I used to do when I only had the same.
Day1. Chest Tri
Dumbbell Bench Press 4x10
Dips( get two chairs, armchair, any two things side by side of the same height that can support your bodyweight) 3x10
Dumbbell Flys 2x8
Tricep Extensions 3x10
Tricep Kickbacks 2x10
Day2. Bi Back
One arm dumbbell rows. 3x8
Pullups(use doorways, trees, washing line, playground if theres one near you) 4x8
Dumbbell Pullovers 2x8
Concentration Curls. 3x10
Dumbbell Curls. 3x10
Day4. Shoulders legs
Dumbbell Shoulder Press 4x10
Front Dumbbell Raise 3x8
Side Lateral Raise 2x10
Reverse Flys 2x10
Dumbbell Lunges 4x10
Dumbbell Squats 3x8
Reverse Lungers 2x8
If you don't know what these exercises are look them up on the link I put up there
This is not the most ideal workout, but there's only so much you can do with dumbbells, if you really want to get serious join a gym, or buy a barbell. Compound exercises are great for beginners i.e - bench press, deadlifts, squats, military press, snatch/power clean.
Don't forget to eat heaps, and get adequate rest between workouts.
Good luck!!