What the length of instructions on these processors will be?
If the ISA is the same, the registers and instructions have the same size.

A CPU can have 32, 64 or wider busses for caches, external memory.

It has no relation with the size of instructions.

It has no relation with the size of integer registers.

You can see variants or extension of the same ISA with 64 bits support : MIPS III, SparcV8 vs SparcV9, x86 vs AMD64, ARMv7 vs ARMv8, Power32 vs PowerPC64, etc... The new instructions are either handled as a different mode, or using different opcodes.

The "size of word" is a term for compilers and programmers (int, long int, long long int...), not for hardware.

Of course, it makes little sense to tie a 64bits bus to a 8bits CPU, or the inverse.