On my calc (depends on the calc) they are together just 200 bytes big, maybe it's possible even smaller.
Dec → Bin
ClrHome Input "Number?",A If A>65535:Then // This is because the calc gives the binair number on 1 line, and if the number is bigger than 65535(2^16-1) it needs more space Disp "Too big!" // Of course, if you like, you can modify it so it can have larger numbers, but I don't think that's really needed. Stop End For(B,16,0,-1 C+1(B≠15→C Output(2,C,0+1(A>2^B A-((2^B)(A>2^B→A End Stop
Bin → Dec
ClrHome Input "Number?",Str1 // This one has no max number, you can fill in binary numbers as big as your screen, whatever you like. length(Str1→C // I use C here because this line takes less space than 2 times length(Str1 in the program. 0→B For(A,0,C-1 B+(2^A)(sub(Str1,C-A,1)="1→B // If part of the number with length 1 on a place (starting at the right) is 1, put 2^A (1,2,4,8,16,32,...) + B in B End B // Something I learned here, don't use Disp B, just B is enough.
I hope you like them, I don't know if anyone else has made them once before (I think so).