Mine loves to beg like that, or at least he did a lot at one time, and I always thought I was feeding the right sizes. Sometimes even after feeding a pretty big meal for him (he's pretty small, only 250g), he would keep looking at me. A big meal means abit larger than his widest part and taking quite a while to push it down. Now and then I would give one more, he would take it.

Then I bought a digital kitchen scale. Oh wow have I been underfeeding him! Maybe it's because he looked pretty skinny back then but at 250g I was feeding him no more than 20g! And sometimes the 18g mice and rats have hips quite a bit wider than his body. No wonder he looked so hungry! So now I like to base it on weight rather than size. Of course if a 25-30g looks wayyyy too big for him to swallow I'd just give him 2 smaller ones instead.