How to compute the output length of randomized encoding (or garbling)? We consider randomized encoding for circuits as a equivalent primitive as the circuit garbling scheme. However, I have a question arised from their syntax, that is, how to compute the output length of randomized encoding?

From the syntax, the encoding algorithm of randomized encoding takes as input the circuit $C$ and an input $x$ and outputs a randomized encoding $\widetilde{C}(x)$, but the circuit garbling scheme handles the circuit $C$ and input $x$ separately, that is, the circuit garbling algorithm takes as input the circuit $C$ and outputs a garbled circuit $\widetilde{C}$ and a garbling key $K$; the input garbling algorithm takes as input the input value $x$ and the garbling key $K$ and then ouptuts a garbled input $\widetilde{x}$ 

does it mean $|\widetilde{x}|+|\widetilde{C}| = |\widetilde{C}(x)|$? I know that the online complexity of garbling scheme gives a bound on the size of $\widetilde{x}$, but what is the size of the garbled circuit $\widetilde{C}$? Can we implicitly regard it as $poly(1^\lambda, C)$
The size of the randomized encoding (or garbled circuit) produced by a given construction will be specific to that construction. (Indeed, there are major efficiency improvements to GCs/REs that are still being discovered every year.) So, to answer your (own) question, it would require focusing on a particular construction's details to answer fully.

However -- Yes -- it must be the case that any garbled-circuit / RE that is output by any probabilistic polynomial-time $\mathsf{Garble}$ or $\mathsf{Encode}$ procedure has size at most $\mathsf{poly}(1^\lambda, C).$ For the procedure to output something significantly larger, e.g. exponential in $1^\lambda$ and $|C|$, is impossible (in prob. poly time). Moreover, the typical constructions that come to mind -- e.g. Yao's Garbled Circuits and Applebaum-Ishai-Kushilevitz's randomizing polynomials -- have size approximately $O(\lambda\cdot |C|).$ (It's likely that whatever you're looking at is 'loosely in this range,' though schemes close to the state-of-the-art will have various fine-grained differences..)